I recently came across this site,
FreeRice.com.It is very nice, because you can learn vocabulary and help end the world hunger. The idea is very creative.
This is how it works: just log-on to the website and play the vocabulary game,
for each question answered correctly 10 grains of rice will be donated by the sponsors to help people living in poverty.
It is a win-win situation - you get to play the game,
FreeRice get the traffics, advertisers get their exposure, and people get the free rice.
If you want to keep your game record, please make change for setting by enter to the options page.
For more details, read their FAQ page.:)