Someone sent me a newspaper article about FreeRice and now I'm hooked. It's a fairly simply vocabulary quiz (with some fancy programming behind it that keeps it at a challenging level for you, whether your vocabulary is at a fifth-grade level or a lexinerd level. Each time you get a word right, the site's advertiser's (the ads are unobtrusive) will donate 10 grains of rice to the UN's World Food Program.
It's as absorbing as computer backgammon and I can even tell myself it's educational and supports a good cause!
What got me started was What's the Word? We Can Help Feed the Hungry, Wash. Post, Nov. 4, 2007. Thanks, MEDH!
Update (Dec. 14, 2007): Now they'll donate 20 grains of rice for each correct guess! It's still a fun game and now you can do a little more good.