Crestfallen put this up on her blog, and I think it's fun (and helpful).
Check out: Free Rice. It's a site that tests your vocabulary and donates rice to the United Nations World Food Program in the process. There are 50 levels of difficulty, and the site claims that very, very few ever get past 48. I peaked out at level 42, but I can only consistently reach level 40.
So, all you logophiles out there... give it a try!