This is my own personal favourite right now
Shamelessly stolen from the newsletter a few weeks back. I showed it to some people, and it produced a hearty chuckle.
The sort of thing that would brighten your day if you saw it:

img.photobucket.com/albums/v16/carlwinslow12345/file.jpg
(sky blue sam, Thu 3 May 2007, 19:34, Reply)

Scratched on the inside of a lift door, Manchester University physics department...
"HOORAY FOR JIGGY PIG AND HIS BALL OF FOAM".

We never did find out who Jiggy Pig was - and since the lifts in question were replaced a few years ago, 'cause they were showing disturbing levels of sentience and free will, this is perhaps the only chance to immortalise this peculiarly cryptic message...
(Happy Idiot Talk now has an alibi for, Thu 3 May 2007, 19:33, Reply)

College
Alot of graffiti about here.
"I LOVE MUSLIMS... I THINK THEY ARE GOOD AND ACE!"
There is a sign on the loo roll holders that reads "Jangro PROFESSIONAL PRODUCTS" which has been edited to "Jangro ROFLSSIONAL PRODUCTS" and on the wall behind the turlet:
"SEX is for cats and chimps, REAL MEN MASTURBATE"
And that kind of lark.
(annhialus hasn't had an accident since, Thu 3 May 2007, 19:30, Reply)

Only in Thamesmead
The area i'm in is well known for it's population of petty-minded chavs, who think that "tagging" something with a nickname is the best way to get popularity. Going to the school there only showed me just how bad it can be - graffiti everywhere they can get away with it, and then some. But two really good examples spring to mind:

1) During one of my english lessons, I was bored and looking for something to do - and my favourite pasttime was correcting the spelling of any graffiti on the desk (like I said, bored). Anyway, this one caught my eye - someone had tagged the desk with not only their real name, but the names of the group of friends they were sitting with. Great - only the time and date was missing from that one.

and

2) One of the biology classrooms, again finding any excuse not to do work, so was skulking around looking at the cockroaches and rats held at the back of the room. Passed a potted plant - you guessed it - where someone had taken the time to write their nickname on each of the leaves of a plastic plant in permanent marker...

I really wish this was the worst of them. Lucky I'm leaving in a few weeks.
(Nykrus, Thu 3 May 2007, 19:27, Reply)

I feel their pain

( crackhouseceilidhband Fuck off back to Mumsnet, Thu 3 May 2007, 19:27, Reply)

Loads of these
One on the wall the urinal is on saying "come closer, it's shorter than you think"

one cheeky sod had scrawled on the floor "Look right", then on the respective walls "look forwards", "look up", and finally "look back" on the ceiling. Behind you the immortal words "WHAT THE FUCK ARE YOU DOING YOU'RE SHITTING ON THE FLOOR! LOOK AT THE FLOOR AGAIN AND CONCENTRATE"

I like it anyway.

"James loves bumgayrapistcockmunchers - call 07959....."

Making Mr Pringles look like hitler.

"Vote BNP" - and with "do a little dance mid-shit, if you" scrawled on top - in the toilet cubicle again.
PHWOAR LENGTH

edit: another toilet one: "mummy, daddy, if you are reading this it is because I didn'T come home. I'm meeting Charly off the internet in a minute in here" with two childsized, pretty shit drawings of handprints in red colour... ouch...
re-edit: and of course who can forget the classic "HAHA! You're fucked now!" on the empty roll of bogpaper.
( morrybyte made a subtle namechange at, Thu 3 May 2007, 19:26, Reply)

on a train
there was a sign saying

"Please give up this
seat if
a disabled person
needs it"

which some one had scratched away to make:

"Please
eat
a disabled person"
(General Arfarfarnarf He who hesitates, Masturbates!, Thu 3 May 2007, 19:24, Reply)

Edinburgh University
On the side of old college, the main building was tagged: Ken More. (ken being both a blokes name, and the scottish for know, ken?).

Also, around the corner in Chambers Street is a fancy entrance with big carved stone urns either side of a door. The one on the left was marked Eric, the one on the right Ern.

Made me laugh.
( littlefish I like cheese, Thu 3 May 2007, 19:23, Reply)

I remember in the bus shelter waiting for the school bus back in '91
and this little ditty was painted inside it:

Some come here to sit and think;
Some come here to shit and stink;
Some come here to scratch their balls
and read the writing on the walls.

Not quite as good as seeing a message painted on the road referring to the local MP:
"David Drew sucks cock"
I've no idea if he does or not, but the road had been resurfaced two hours later!
( M3rx likes the Asian ladies, Thu 3 May 2007, 19:22, Reply)

most graffiti, in my opinion, is shit
because its just some chavved up arsehole assuming that we care that he learnt to write, and that we know what he means when he scribbles
"P3rs1l massif 4 eva" on a tube train. However, I have seen some particularly witty graffiti, namely:

Plan Ahea
d

"God is dead" Nietzsche
"Nietzsche is dead" God
(tarbin tarbis, tarbit, tarbamus, tarbimus, tarbant, Thu 3 May 2007, 19:21, Reply)

Pub poster
Not graffiti per se, but in one of my favourite haunts in York, a poster offering food deals reads something like, "these offers may be forgotten, changed or lied about at any time." Always makes me laugh.
(badfox1 wonders who he's played by this episode on, Thu 3 May 2007, 19:18, Reply)

slept through the seventies. and the eighties. the nineties too.
Appeared in the last few years on a wall alongside the railway terminal...

"Yanks out of Vietnam"

I guess some sixties relic finally came down off whatever it was they took, and got back to business.
(bollox, not again, Thu 3 May 2007, 19:16, Reply)

I didn't know they came from factories...
Outside a coach company on my route to college every day for 2 years, some local chavs had put up an ingenious sign proclaiming that this was, in fact, a "GAYBOY FACTORY".
Every day I wondered if it was a factory where gayboys worked, or where they were made on some kind of production line. And every day it was still just as funny as the day before. :-)
(the_n00b, Thu 3 May 2007, 19:13, Reply)

Found this stuck to a wall
on the stairs from Tower Bridge to the riverside the day that it happened:


*slightly tweaked for clarity
( NomDePlume fo shizzle my nizzle old bean, Thu 3 May 2007, 19:13, Reply)

I almost forgot.
BIG FRENCH WILLIES!


You can go and see it for yourself if you want. It's in Manchester, at that retail park opposite the Trafford Centre. On a disused building at the far left hand side. Go on, go and have a look. It's a lovely day and everything. You might as well.
(spangledonkey I've got a mouse. His name is Gerald., Thu 3 May 2007, 19:13, Reply)

Showing My Age?
I think it was in Brentford where "Elvis Lives on in Our Memory" was altered to read "Victor Lives on in Our Memory"
(onefishtwofish Redfishbluefish, Thu 3 May 2007, 19:13, Reply)

Criminal Genius
Picture the scene; its the middle of January, approaching midnight and bloody freezing. I've spent the entire night drinking in the pub with a few mates watching the pretty snowstorm outside. They've finally chucked us out and we're weaving our way home, stumbling and slurring through the, now gently, falling flakes. Suddenly one of my friends is bent double laughing loud enough to wake the entire street. I ask him what's so funny but he can't stop laughing long enough to tell me, he can only point and continue giggling.

My gaze follows his finger to see a battered blue van sitting in a driveway. It's covered in snow, except for on the side where a childish finger has drawn the following....


dad is a fartt
( Gleeballs is mostly made out of chocolate and hate., Thu 3 May 2007, 19:11, Reply)

Toilet
Our school toilet doors had a good 6-inch gap between the floor and them. Some wag had draw a downward arrow towards one of these said gaps with the strange device "beware of the phantom limbo dancer" accompanying it.
( Fray Brentos *mega-belms*, Thu 3 May 2007, 19:10, Reply)

Cash machines in manchester
Several cash machines across Manchester, mainly near Oxford road station.

The penmanship was exceptional and the level of upper class bile really hit home.

"David wallis* is an insignificant wretch."

"I spit on David Wallis"

"I consider David Wallis to be the scum of the north"

*- I think it was Wallis, but I may be mistaken. It was definetly david though. Someone else must have seen these.
(J.T Squid, Thu 3 May 2007, 19:10, Reply)

In Santa Cruz Ca.
There's a dual carriageway that merges with another road, but due to the direction change it's a sharp bend that tightens.
The warning sign says "Curve Sharpens" but being a pretty cool, laid back, hippy kind of place it got changed one night to "Curve Happens". Like, Yeah man.
( Grindstaff, Thu 3 May 2007, 19:08, Reply)

Historical... and money saving?!?
Some scumbag had scrawled
"Hitler was right!" on the top of a table of a train I worked the other night.

Underneath, someone had added
"...to change his gas supplier. Uswitch.com"
( HSDriver was probably still at work on, Thu 3 May 2007, 19:08, Reply)

One door.
Rough pub somewhere in London. God knows where, I don't do geography. Going down the door, a load of badly scrawled anti-semitic and generally racist stuff. Then, about halfway down, in really elegant script:

'I fucking LOVE Dylan Thomas.'

Then a bit more stuff about nazis and hating jewish and black people, before right at the bottom:

'Super Clive Mendonca!'
(spangledonkey I've got a mouse. His name is Gerald., Thu 3 May 2007, 19:06, Reply)

Westminster uni, fire exit
'Door is alarmed'

Someone crossed out the alarmed and replaced it with 'terrified'

Simple, but made me chuckle.

You know when you start giggling to yourself on a group interview of 20 or so students, it really doesn't look that great...
(Big Spoon Baby Balloon hides in corners, Thu 3 May 2007, 19:06, Reply)

elfinpunk
photo evidence please
( mrlogic is copying your beard, Thu 3 May 2007, 19:06, Reply)

Inside the door of the gents on a campsite in Ireland
I read "Susan, if you're reading this, it's all over between us."
( no cake for the impuritay Patrick Stewart hasn't aged since, Thu 3 May 2007, 19:04, Reply)

East End Pub Toilet Cubical Wall:
This Pub is Shit

Then underneath, a graffiti arguement broke out over a number of weeks, with more comments added underneath:
"No it's not"
"Well maybe on a Thursday"
"I has actually gone downhill slightly after a recent management change."
"I like reading this"

.etc.

Went on for so long the back of the door was nearly covered, with threads being added to the arguement. I was quite impressed and always used to pop in to see how the 'messagedoor' was going!
( HSDriver was probably still at work on, Thu 3 May 2007, 19:03, Reply)

Chemists are cool. OK!
From a toilet roll holder in the cambridge university chemistry department...

Potassium ethoxide rules C₂H₅OK!


length? a few angstroms.

woo! first post!
relurks
(hedgepig sucks moisture out of wood, Thu 3 May 2007, 19:02, Reply)

Sex Change Street
I work in Romford, and our office backs onto an access road called Exchange Street.

Someone has scrawled an "S" in front of Exchange.

I thought that was quite clever for the chavs of Romford.
( SeasonTicketless is too often the statue, and rarely the pigeon., Thu 3 May 2007, 19:00, Reply)

The Crow's Nest
Technically, it's my Dad's favourite graffito.

Apparently, when he was a student at Durham University in the late
1950s/early 1960s, somebody painted some giant bird footprints leading
from The Crow's Nest public house into the public toilets down the road.

Genius.
( finnbar has officially retired, Thu 3 May 2007, 18:55, Reply)

Ohhh Another...
Admittedly at partially my own hands...

Drawing the word COCK in 40 foot high letters (the tide goes out by about 2 miles leaving a lovely blank canvas), on Brean beach underneath the flight path for aircraft into Cardiff and Bristol airports. Visible from the air.? Visible from the fucking moon.!

EDIT:- No I don't have photo evidence, sadly. If any one from the US space agency or some Communist country with a large spy satellite and back ups from the Bristol area from 1996 is reading this then let me have a copy, please.!!!
(elfinpunk Touching Cloth Since, Thu 3 May 2007, 18:54, Reply)