Click for charity
Click once per day
The hunger site
This is the best known "click for charity" site. It actually consists of "The Hunger Site", "The Breast Cancer Site", "The Child Health Site", "The Literacy Site", "The Rainforest Site", and "The Animal Rescue Site".
In addition to clicking for charity, each site has an online store where you can purchase items with some of the profit going to charity.
The Hunger Site and related sites
Click many times
FreeRice
Here's one that debuted in November of 2007, freerice.com, They have a vocabulary quiz, where you select from multiple choices of word meanings, and rice is donated for each one you get right.
The words have a very wide range of difficulties, starting off with easier words, and adjusting the difficulty of the questions to match your answers. There are 50 levels of difficulty, although they say in their FAQ that rarely does anyone advance beyond 48. I can believe that, I've never managed to top 46, and 44 or 45 are far more common when I try it. I'm pretty much guessing based on root words at the higher levels!
Try it, it's fun!