this is actually really addictive and possibly what caused me to fail a lot of my uni exams. It's a bit old, and the forum seems to be inactive nowadays, but nonetheless its worth a go:www.jaor.net
(just another online riddle)
The basic idea is you look for clues hidden in either the image/text, HTML source or similar and then alter the URL filename to the "answer word" to get to the next level. It's surprisingly addictive. Sometimes it gets rather cryptic - which makes it even harder.
Advice though - everytime you have to enter a username/password to proceed to the next level, write it down. Also bookmark every level.
To get to level 99 took me about 2 weeks...