Cory has a great article about biking:
http://madvilletimes.blogspot.com/2008/04/real-people-power-get-on-your-bike-and.html
I have been (trying) to ride everyday. The rain is stopping me today, but Have been able to do it everyday so far since last Monday. It’s about 7 miles one way on the bike trail. So I don’t have to wade through the traffic in SF (which majorily sucks) because we have incredibly apathetic drivers. So it gets to be a nice workout, because I don’t have to stop for lights. I also have been saving $$$ on gas.