I have recently discovered this site and it can be very addicting! You do have to download their toolbar if you join but it doesn't have anything bad with it.
The mention of a "tool bar" turned me off, and because they make you download a tool bar they probably deposit spyware and tracking cookies on your computer, so I tend to stay away from sites that do that.
I used it as a firefox addon at work a while back (it didn't seem to come with anything bad) and got addicted but it soon wore off and I never bothered installing it here at home