I have to say that the most used tool I have is probably my die grinder. I have several of them, and my favorite ones are the small air-driven 90 degree models. I use Ingersoll-Rand and they give me very good service and aren't that expensive. Models from Harbor Freight are cheaper, and have pretty good life as well.

The die grinder is a very versatile tool. I have 3" cutting wheels for it, in 1/16" and 1/32" thicknesses. The thicker ones are good for grinding, such as smoothing welds, and the thinner ones slice body panels with ease. I use them a lot for body and fabrication work, or for cutting off stubborn bolts, etc. It makes short work of a lot difficult problems. I also have a collection of diamond-cut solid carbide burrs. These things come in really handy for enlarging holes, rounding inside corners, and de-burring cut-off parts. I couldn't have done some of my fab work without these burrs.

Sometimes I use drum sanders with my die grinders. They don't last very long because of the high tool speed, but they come in handy now and then.

I also have one straight die grinder that I use when a 90-degree one just doesn't work. It comes in handy in some places, and accepts the same burrs as the 90 degree model. I don't like to use the straight one with the cut-off wheels because I feel I have much less control with it than with the angled one.

Get a couple of die grinders and put them in your toolbox. You'll wonder what you ever did without them.