A few more things I thought of:
- Tire pressure gauge. Not the kind with the slide that that gets pushed out, since they aren't very accurate. You want one with a dial and needle. Sure, your TPMS can let you know when the tire is low, and on the newer vehicles it can even tell you the exact pressure reading, but that doesn't update quickly and it's pretty useless while you're trying to air up or air down. Most gas station air pumps I've come across don't have a gauge either.
- Tire deflators. You can buy one, or you can make them (there was a thread on here for DIY deflators). If you are going to buy, the ARB or Currie deflators are nice since they have a built in gauge (killing two birds with one stone) and they unscrew your valve stem to let out the air fairly quickly.
- Tire repair kit. If you have a really bad day because you've had two flats on the same trail, this could be the difference between getting off the trail or hiking out.
While we're at it, what about other spare parts that may be needed on the trail? Drive train components like Spare U-Joints, axle shafts and drive shafts are the ones I've most commonly heard of being needed. Any others anyone would recommend?