Learn. Have Fun. Feed the Hungry.
This is a GREAT example of viral social good. Check it out: freerice.com. You take a multiple choice vocabulary quiz. For each correct selection the site donates 20 grains of rice to the United Nations. It's positively addictive!
From a web 2.0 standpoint the two things I'd love to see this site add are: 1) a quick link allowing site users to forward it to their friends, and 2) a widget allowing users to compare scores against their friends'.
Organizations can learn from this: it's fun, it's fast, it's rewarding. It gives immediate feedback. The site design is clean. And since launching in October 2007 they've gotten 500,000 users / day. (See short video.)
I donated 6540 grains of rice this before I ran out of time. My favorite new word is 'omphaloskepsis', meaning 'navel contemplation'. Niiiiiice!