and another
Just remembered one more. We used to play one up on eachother at work all the time. I used to work in email and it started by faking eachothers email addresses and inviting the whole team out for drinks or spamming eachother with fake subsriptions to gay prawn sites or NAMBLA etc.
There were several of us and it seemed a good way to keep up morale (I was boss - and a bloody great one!).
One guy and myself would occasionally get into a bit of a war with it, but all in good humour.
Here's a few I can remember.

Sello taping over the mouse ball (so it doesn't work) and the earpeice of the phone.
Changing your caller ID (that only the outgoing number see's - you don't) to various things - Gandalf, Tom Cruise, Gaylord Fokker, Shirley and shaggy are only a few of hundreds that I can rememeber.
Everyone had two PC's, so we'd swap the keyboard and mouse cables over.
Putting weightlifting weights into someones rucksack. We added one a day for three days before he realised. The guy lived near Brighton and worked in London and had been carrying it there and back every day. After he went nuts over that we started adding tech manuals (you could beat an ox to death with some of them). After he started checking the main pocket of the bag each day before leaving, we stared filling up all the other pockets with paperclips, nuts and bolts etc. He left soon after that.
Setting Windows to play that bloody awful frog thing on start up, then with every window activation, every application startup and every notification.
Taking a screenshot of the windows desktop, setting it to be the desktop picture and then deleting all the real icons. Took ages to figure out the first time!
Taking a screenshot of the windows desktop, then printing it out in colour and taping it to the glass behind the anti-glare screen.
Whilst the victim was on holiday, spending a casual two weeks creating a sculpture with their chair danging over the desk using about 600 elastic bands, creating a web-like effect all round the desk. The overall effect was like a web that dangled from the ceiling and engulfed the entire desk and it's contents. It was a work of art.
One guy (who wasn't on our team) received an anonymous package every few weeks containing all sorts of crap that had been lying around our desks. Screws, bits off of broken pda, cables, all manner of crap. We used to keep an internal envelope for anything we didn't think we needed anymore and, when it was full, send it to him. We could see his desk from ours and would get great joy out of seeing him go slowly mad over who was doing this.
We did this for eight years. They still do it now in my honour (I'm not there anymore).

One time, late on a Friday before going away for two weeks I was there on my own waiting for a call. I was bored, so taped over this guys phone and mouse, then changed his keyboard settings to be French etc.
I forgot all about this when I got back, cos of the superb holiday I'd had. When I returned, he was away for two weeks - so by the time I saw him again I'd totally forgotten all about it.
Anyway, about a week later we're down the pub and he tells me.
He'd seen my umbrella lying on the desk, so had opened it up and filled the inside with the contents of several hole punchers and as much ripped up tissue paper as he could fit. Then he'd wrapped it back up and put it back on my desk. He'd assumed that whilst he was on holiday, I'd been out in the rain and been had by the trick. Except I hadn't.
The day before, he'd worked late and it was pissing down as he wanted to leave. Not having his own umbrella with him, he grabbed mine ran outside and opened it up.
Yep, had by his own trick. It took him ten minutes to explain this to me as he was laughing so hard that snot was coming out of his nose.
(EvilMeister, Wed 2 May 2007, 1:29, Reply)