1. Vacuum gauge. They can tell you quite a bit.
2. Spark tester. I have a Lisle pen-looking thing with an inductive groove that you just hold against a plug wire, and if it's getting zap, the light flashes.
3. Infrared pyrometer (thermometer). Not as precise as a contact probe, and my cheapie doesn't have emissivity adjustment, but it's great for comparing one temp to another, or seeing if a temp is just way out of wack. Good for checking the temps of the header tubes when the engine is running (look for the way low or high temp tube). Also good for checking temp at the rear of a catcon vs the temp at the front. Check the temp of your front or rear diff housing, or your transfer case housing temp. Use your imagination!
4. Fuel pressure gauge. Look for out-of-range pressure.
5. Tire pressure gauge.
6. Timing light, maybe one with a tach and dwell meter built in.
For newer vehicles, or older ones that still have an ECM:
Code reader. You can get a decent cheapie that just reads trouble codes, or spend more for one that reads both OBDI and OBDII codes (and possibly CAN) as well as deciphering the codes and telling you what they mean in plain language, and has all the connector cables for different makes. Only worth it if you need it for one of your vehicles, or if you just have some cash to burn.
