I added a usb extension cable for easy access and microphone/headphone extension cables so I could have a lot more freedom to move around the office. What computer accessories do you find useful that you think aren't commonly used?
These let you take an internal hard drive and use it as an external one, with a power switch and USB connector. I found one that accepts SATA and PATA drives, and it helped me a lot with migrating files from one computer to another (with different internal drive connector types). Now my old drive serves as an external backup.
I'm not sure how unusual this is, but it was news to me.
Not exactly Accessories, but...
Every time I buy a disk I go back the next day to buy data and power cables/connectors, and a bracket to mount it in a larger drive bay.
I keep a straightened paperclip for stuck CD/DVD drives. And a Leatherman - no idea how many computers that has helped me work on.
I have an ATEN KVMP switch (similar to this) so that I can switch the keyboard, screen and mouse to one of the physical computers whilst being able to move the headphone connection to the same or a different one.
I, too, have USB extension leads from computers that are not on my desktop to provide Type A, Type B and micro USB connectors on the desktop.
The headphone lead from the ATEN KVM is an extension lead so that I can easily plug in either the headphones or a lead going to an appropriate input socket on an amplifier to allow easy switching to good speakers rather than headphones when appropriate.
A final odd accessory. As I use a USB keyboard and mouse but the BIOS on my main desktop machine does not recognise USB (yes, it is that old), I have a Fujitsu Siemens numeric keypad plugged directly into its keyboard PS2 socket to select boot up options, when needed. Even though my keyboard doesn't have a numeric pad I don't use the numeric keypad for its intended use.
Wireless headsets! I've been looking for a good wireless headset for a long time and there are only a few out there. I think logitec just released a pretty good one though. I have a bluetooth headset that is pretty nice but the sound quality suffers for music and the range isn't the best.
I think a UPS is the best investment that I bought for my home desktop and not a lot of normal users have those but they're gaining in popularity.
The most used accessory I have is a IDE/SATA to USB cable to easily hook up and restore harddrives that fail. Many of our bookkeeping department have computers manufactured in 1992. We recently replaced about 70-100 of the older models, but the bookkeeping department wanted to keep their oldie goldies. So now I'm stuck with tons of spare parts. I guess as long as they can put up with it, I can keep replacing harddrives. I haven't had many processors/power supplies fail, just harddrives.