Somewhat ironically, as it has just been mentioned - in second year I started volunteering at a local animal shelter, and quickly fell in love with it. It became pretty much the only thing I did. So I started a society for it as well. We encourage our members to go there and help directly, either in cafe/hands-on help/shop work, or we gather volunteers for outside events such as marshaling races, for which the organisers will give the charity lots of money. And on campus we fundraise through quizzes, crawls, bingo events, cake sales, anything else we can do. It's pretty hard to keep it going sometimes but it's been so important to my university life.
I'm also on my college netball team, if that counts, and I've had ties with the Pokemon Society, which I/my partner also helped found in first year, and a couple of others music/game based. But the animal charity society is my baby, I'll be so sad to leave it and pass the presidential baton over... partly because I really fear for its future. Eek