If you're running out of USB ports on the back (or front) of your computer, free up some space: Stick wireless dongles (for your mouse or keyboard) inside your computer with one of these cheap little header adaptors.
These adaptors -- whether in compact form or cable form -- allow you to plug a USB device directly to the header on your motherboard, rather than one of the ports on the back. Since lots of motherboards have more headers than you'll actually use, it's a good way to plug in more USB devices without taking up any ports. You could use them for wireless mouse and keyboard receivers, a USB security key, or even a boot drive (for systems running something like FreeNAS).
After seeing these on Linus Tech Tips' "Handy Tech Under $100" series (embedded above), I realised I had one of these in my giant box of cables and never realised what it was -- so it might even be worth checking your box to see if you already have one. It's a small but handy item if you have a lot of USB devices. Check out the links below to see the different versions you can buy.