Sound pressure level meter
Now let's get a little exotic. At around $260, the Extech 407750 sound pressure level meter is probably the single-most expensive tool I regularly use (though a variety of affordable SPL meters exist, ranging from $30 to $70). An SPL meter is useful for measuring sound output from a PC, which is essential when your main goal is building a quiet model. The Extech has a tripod mount, so you can easily point it where you want it, set the distance, and lock it in place. I typically use A-weighting, slow response settings. I've also used this meter when calibrating the room where my HDTV and home theater gear live.
Bottom line: your favorite tool
Whatever your DIY predilections, using the right tools will save frustration and time. However, having good tools doesn't just save time. The old adage about how everything looks like a nail applies here. If you're trying to use a tool that's not quite right, then you risk damaging hardware irreversibly. Having stripped my share of screw heads and twisted off more than a few connector shells, I've made plenty of mistakes using the wrong tools. So once again, take a look at that pile of parts on your table, and understand just what you need to get the job done efficiently and correctly. Your reward will be a POST screen seconds after you press the power button the first time.