One thing PC gamers can never get enough of is disk space. PC games can take up a ton of room on your hard drives, with 60+ GB installs fairly common these days among AAA titles. But when it comes to budget gifts, you can get a lot for your money these days with hard disk drives. A quick Internet search will show you that a 1TB internal HDD runs about $50, with external drives of the same space costing just a little more. While HDDs are slower than SSDs, the sheer amount of storage space per dollar is much cheaper: a 500 GB SSD will run you about $160.
What I do and recommend for PC gamers is to install their OS on an SSD and maintain a large HDD for general storage. You can move your games over to the SSD when you want to play them to cash in on those sweet load reduction times. So don’t think that gifting a new HDD is a waste of money!
While not necessarily solely related to gaming, USB flash drives are also great portable storage devices that make great stocking stuffers that are relatively cheap. Plus you can buy all sorts of cool gaming themed drives, like this awesome Super Mario Land Game Boy flash drive. If you get a large enough drive, you can even install games like World of Warcraft onto it to play from another computer and keep your custom interface configuration.