www.ouverture-facile.com
yes the riddles are in french (all of them are flash animations), but they are sooo interesting!
I've been stuck for three days with my roomate on them (and we are in Paris, so you can see how addictive this thing is! But we are going out and enjoying the city, don't worry)
The riddles are in flash and the site has a forum were users discuss the levels!
A must for riddle-lovers!