Tools I have found handy
 

and at least one I wish I had but have not had the need bad enough to justify the expense.
That tool I have used the most is a fuel pressure gauge. It is a tool I never had the need to use with a carbureted car but has proved to be a valuable addition to my tool box in diagnosing a fuel injected car.
My next purchase will be something to read codes out with. Up to this point any car I owned with fuel injection had a way to get the computer to display codes for diagnostic purposes with out having to use a scan tool. And in the mean time Scan tool prices have come down and become more versital. I am sure that the next major computer problem I am trying to diagnose will require the purchase of a scan tool and once done I will not regret having purchased one.

I bought an OBD2 code reader and it is a wonderful piece of equipment. We have three vehicles and it's paid for itself twice over

You are right there -- and OBDII scan tool is worth its weight in gold ...

I have an older Actron OBDII scanner. I take it with me when I go look at a used car. If things show up that will be a costly fix, I walk away. If it shows a few minor things, then it helps negotiate a lower price.

I use the Actron too. Great tool.