Weffriddles Kill some time by working your mind.

My friend turned me onto this website weffriddles.com. It's not those silly riddles that you used to read in your Highlights magazines as a kid. The answer to each level is the url for the next level, so it's less likely that you could enter a random letter or word and get to the next level. In fact, the answer isn't always necessarily a word or letter. Though the first few levels are rather simple, the further your progress requires you to do a lot of "out-of-the-box" thinking and go through multiple steps to reach the next level. Right now, I'm on level 43. They've got a forum to give you hints and push you in the right direction, but they try not to give spoilers because the whole fun of solving the riddle is figuring out the answer.