My brother dared me to try this site, and now I'm hooked!
www.amnesya.com (http://www.amnesya.com)
It's sort of like the most complicated online puzzle/riddle game I've ever seen. One warning though, it does require a little knowledge of computer programs to use (ex. in one puzzle you have to change the contrast in a picture after downloading it to your desktop to find the answer). The tutorial is a must, but it's definitely addictive!
I just made it through the tutorial. Wow, what a site! I doubt I'll make it very far, but we'll see. =)