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Sure, you can spend big money on a PSU tester, but for confirming a dead PSU I find a cheap tester is handy. Also, if you have a few hard drives handy, you can add some load to the PSU by connecting them up before you test. My go-to device is the FrozenCPU tester. It's cheap but reliable and does what is says on the tin and not bad value for a little over $20.
If you want to take power troubleshooting to the next level then you can get your hands on a decent multimeter. If it's something you're going to use occasionally then a cheap one will do fine, but if you want something that will last you years then I'd go with the Fluke brand.
Broken/chewed up screw remover
I find that jobs get exponentially tougher when someone else has had a go at fixing something and in the process caused more problems.
One problem I come across every so often are chewed up or broken screws. Usually they're as a result of someone being too enthusiastic with a poor tool, but other times they're because someone's taken a powertool to the screws.
iFixit have two great tools to deal with such problems. The first is Neji-Saurus – the screw extracting dinosaur. It's a crazy name for a fantastic tool that can grip screw heads, bolts, or nuts allowing you to twist out even the most damaged fasteners. It might seem steep for $30, but it's a real lifesaver.
If you need to tackle screws that have had their heads sheared off completely, the precision screw remover set is worth a look, and can help get you out of a jam.
Tweezers/magnetic pick-up tools
To paraphrase Vincent Gambini from the film My Cousin Vinnie, repairing a PC is a procedure; like rebuilding a carburetor has a procedure. You know, when you rebuild a carburetor, the first thing you do is you take the carburetor off the manifold. Suppose you skip the first step, and while you're replacing one of the jets, you accidentally drop the jet, it goes down the carburetor, rolls along the manifold, and goes into the head.
You're then in a world of pain.
While there's no head to drop screws into in a PC, they can end up in awkward, hard to reach places. And if you don't have the right tool you can end up having to up-end the entire PC to get it out.
This is why I like to have tools to help out. First on the list are tweezers. While you can get plastic ones, I prefer ESD-safe metal ones because they are stronger. Whia has a good range spanning different shapes and sizes.
I also like to have a magnetic pickup too, not so much for working inside a PC (it's not the magnetism I'm worried about but more the way that the head can be drawn to wards a metallic object with enough force to cause damage) but for picking up screws that have fallen on the floor, rolled under a desk, or dropped into a dark crevice.