FreeRice
Thought I'd help promote this website - FreeRice: http://www.freerice.com/index.php.
You play the quizzes and for every correct answer, they donate grains of rice to help feed people in third world countries! It's a great concept - you can learn new trivia and have fun, whilst helping those who are in need.
I only just discovered it today and thought I'd share it with you - the more people who visit it, the bigger difference it can make! :)