Socially good things to do with your computer:

Lifehacker had a post a within the past few weeks about socially responsible uses for your computer. It includes things as simple as letting your computer calculate protein folding or other heavily compute intensive tasks when it isn't used heavily, to networking sites that make it easier to get away from the computer and involved.

I discovered the Breast Cancer Site/Hunger Site a long time ago and fell out of the habit of visiting daily to make the donations, but definitely a worthwhile cause that only costs a few seconds a day.

Some great suggestions in the comments as well.