There are many useful gifts that you could purchase for the Linux user in your household; or even a Windows user, although Linux Mint; Arch Linux and Fedora are better choices on the desktop than Windows if we are talking about the Windows 8 release.
- New hard drives. A new hard disk drive will do wonders for data safety; a hard disk drive can fail and they could lose all of their data. If you purchase a new hard disk for them and they can have the peace of mind that comes with the backup process then you have done them a good service. I recommend Seagate green drives. I use this brand and they are very reliable.
- Good quality USB flash drives. I have just had a USB flash drive fail on me; so I am stressing that it is very important to make sure that you buy a good quality brand of USB flash drive to make sure your data is safe. if you use the aforementioned new hard drive to back up the contents of the flash drive to then the data will be safe. Here is a 128 Gigabyte model: link.
- A nice new screen for their computer would be a good gift at the boxing day sales. A 1920*1080 resolution flat panel monitor would be the perfect gift. This model on Newegg is going for a very good price: link.
- A nice Linux related T-Shirt. This T-Shirt has a Linux cheat-sheet printed on it upside-down so that you can glance down whilst working in the Linux terminal and look for the command you are seeking. Linux cheat-sheet T-Shirt: http://www.thinkgeek.com/product/dabb/?srp=1.
For more gift ideas look here: http://store.linux.com.