Notpron
Notpron is billed as "The Hardest Riddle on the Internet" - and it is a challenging and strangely addictive "game". The general idea is that you start at level 1 and have to advance to the next level. The question is "how?". The answer changes every level. Sometimes the clue is in the music that is played, sometimes in the picture, sometimes in the HTML code. Sometimes all three. Sometimes something else. But in general, the game is fair and you appreciate the puzzles once you solve them. There is an active forum that provides hints and an IRC chat room that is less helpful.
I have been playing this game off and on since October and am currently at level 109 (it is actually level -26, but that is because the levels go from 1 to 82, then 0, then -1, -2, ..., -44 and then there are ten levels with greek letter names. According to the site, 7 million people have visited, but only 92 have finished. I will finish this (I hope), but probably not until the Spring.
Definitely something I would recommend to those with some time and looking to challenge themselves. One hint, take notes of how you solve each level, it will be useful for level 16. If you can get past level 16, you have a good chance of going much further. And, if you get stuck anywhere before level -26, drop me a line and I will help you out.