I always carry a jack, jack stands, breaker bar with extensions for duallies, couple screw drivers, socket set, heavy duty wire cutters. I had to change a starter in an autozone parking lot once and it was easy with all the tools I had on hand. Ever since, I won't leave home without them.
Edit: Also a harbor freight 12v compressor with a cigarette lighter extension cord (from home depot) for the air bags and tires.