In response to the question below, there is only one tool you need to work on computers: a hammer.
Ask Slashdot: Server Room Toolbox?: jandersen writes “I am the system manager in charge of a smallish server room (~50 servers, most in racks), and I am going to buy a set of tools; but first I want to hear what other people think would be a good idea. Certainly a range of good quality screwdrivers — slotted, Phillips, Pozidriv, Torx. But what else? Tape measure? Spirit level (for aligning the racks)? Any meters or cable testers? A wood lathe? I can probably get away with a budget of a few hundred GBP, but there ought to be some mileage in that.”
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Not just a hammer, a big hammer. One that gets the job done in one swing. Like this one.
All kidding aside, my selection of server room tools can be found here. Most servers are designed to require few, if any, tools for service, so most of what I think is essential is for cabling and such.