Go here
homepage.ntlworld.com/robin.d.h.walker/

and click 'Cable Modem Simple Speed Test'

I find it to be very accurate.
( chobb ,ø¤ºGREATº¤ø,¸¸,ø¤ºRAPISTº¤ø,¸¸,ø¤ºRECOMMENDEDº¤ø, Mon 11 Jul 2005, 22:19, archived)

heheh
I like my 4th test:

4th 128K took 0 ms = Infinity Bytes/sec = approx Infinity kbits/sec
( Hidden Mouse, Sleeping Kitten Gromada? Gromada!, Mon 11 Jul 2005, 22:28, archived)