In my computer bag wherever I go.
I found out the hard way how important these babies are. Last March I got snowed in where I got to college and had to get a hotel room for the night. I am a computer science student, and the practical part of my senior design class was really getting started then so I really needed my computer to be in good working condition.
Anyway, I was snowed in at a Super 8 30 miles from home (I commute to school) with my 3 day old Macbook Air when it died a very sad death (turned out to be a hard drive failure). I didn't have the discs with me, so troubleshooting with the Apple rep was basically impossible.
So now, since I am on the road so much, they have a nice home in my computer bag.