This may be old news to a many, but I found this last night and managed to hooked on it for 3-4 hours until I had to go to sleep.
Many of the puzzles are pretty hard, though some are a little too abstract to the point of being stupid. Still, I had a lot of fun.
http://notpron.org/notpron/
I've heard of that one. I think I made a topic a few years back mentioning MaLa's site (http://www.3564020356.org/) as well. notpron is hard, but MaLa takes the cake. You have to solve a riddle just to register in the first place.
Another riddle site that I got through (A friend got a little too obsessed with solving it :^_^': ) was http://www.weffriddles.com/ .
It's a pretty old site, but chances are all these other websites are pretty old too. :p
They are. Me and my old roomy used to do MaLa and notpron back in '05.
weff made me so pissed during java class years ago in highschool.
Here's a couple more if anyone is interested:
Oddpawn (http://oddpawn.com/)
Zest (http://zestriddle.ovh.org/zest_riddle/)