Usually nothing but the cell phone.
If going on longer trips, an extra fuel container as I have ran out of fuel before. A gallon or two of water if the car or driver overheats. A leatherman multi-tool, small flat blade screwdriver, 1/2" and 9/16" combination wrench (minimum). Sometimes I just throw the Craftsman tool kit in, like this one on sale for Fathers day:
http://www.sears.com/craftsman-450-piece..._450mechanicset

Jumper cables, about 20-30 feet of 12 to 14 AWG wire. A 1" wide tie-down ratchet strap.