10 Amazing Peripherals You Didn't Know You Needed
Kelly Forceps (Hemostat)

Ever find yourself needing to attach a jumper or something like a fan power connector to a motherboard, but your fat fingers can't quite squeeze into the available space? Enter the Kelly Forceps, aka a Hemostat. These are available and either medical supply stores or electronic stores. I think I got mine at a Radio Shack.
These are better than needle-nose pliers for the job of attaching tiny plugs or fishing a screw out of a small crevice, because they can clamp shut. That means they can hold said tiny plug rigid while you're trying to insert them. I don't use this daily, but it's saved my bacon on multiple occasions. At the very least, it can minimize the need for serious PC disassembly if you just need to attach a power connector.
AA Battery Charger

I use Logitech cordless mice and keyboards, which use AA batteries for power. In particular, the G700 I use tends to eat batteries for lunch. If I used alkaline AA batteries, I'd quickly go broke and have to give up my monthly MMO fees. So I keep my mouse fed with rechargeable NiMH batteries using this device. (Yes, I can connect my G700 to the PC via a USB cable to charge it. But that sounds so…gauche somehow.)
Of course, those batteries also power my Nikon external flash units, various remote controls and those LED flashlights, so it's not like I'm keeping this around just to charge mouse batteries.
Adjustable Keyboard Tray

Whether you use a sitting or standing desk, an adjustable keyboard tray makes typing so much more bearable. I actually use slightly different heights depending on whether I'm typing or gaming. This Workrite model weighs about 20 pounds, but the spring-loaded mount makes adjustment easy. You can also slide it under the desk surface when you don't need it.
Smart Phone or Tablet

If you've got one PC that's your main go-to system, and it crashes, how do you figure out how to troubleshoot it? In my case, I fire up a search engine on a smart phone and start trolling for solutions on the web. I can do the same with my iPad, but that tends to live upstairs, while I always have my phone with me.
So while a smart phone can be your music player, let you play games while you're standing in a checkout line at the grocery store or read ebooks, it's one of the more useful troubleshooting tools. It can even be used as an ersatz flashlight in a pinch.
Power Screwdriver

What you see here is Milwaukee 6546 cordless screwdriver. It's my most used tool. In fact, I use it so much, I have two of them, as well as backup batteries. While it's only rated at 2.4v, the amount of torque this thing can produce is startling.
It also costs about $90. There are other, less expensive cordless screwdrivers out there, but the key thing you need for a cordless screwdriver to be useful for PC building and upgrading is an adjustable clutch.
Let me be clear: do not use a cordless screwdriver on PC gear unless it has an adjustable clutch. I keep the clutch set to the minimum setting most of the time, so I never strip screws—even those awful potmetal screws and mounts in most PC cases.