I found a link to this site on someone's journal. I have been enjoying myself, so I thought I would share.
Free Rice It is set up the same way as the breast cancer/hunger/literacy/humane/environmental click a day site, but with a wonderful twist. It is a vocabulary quiz. In order to donate (100 grains of rice per click) you must correctly define a word. The site judges your ability and adjusts the difficulty as you click. I've found it a pleasant way to waste some time.
Go play!