Addictive Games
These are pretty fun and once I get started, it's so hard to stop!!
The ESP Game - labeling images...the words you come up with have to match something your anonymous partner types. Get as many pictures as possible in like 2:00. Unfortunately, you have to sign up for an account.
20Q - play 20 questions against their AI. It's really pretty good! But I've beaten it several times. :) There are several categories of 20Q games too. It's horrible at the Name Game (I think that's a nearly impossible game for it to win). But it's pretty good at Harry Potter. It uses the answers of all players to determine what to expect for each thing.