Got a spare PC lying around collecting dust? You know? That old Windows machine that used to serve you well but has been collecting dust puppies in the shed the past few years? Why not put it to some use.

Clean it up, sell it on eBay.
You would be surprised how many people are on the lookout for older components. You might only get 50 clams for it, but that better than nothing.

Donate it to a not-for-profit organisation or local day-care centre.
Not all charity organisations are granted luxurious IT budgets, and donating a computer can make a big difference to their day to day operations.

Make a backup server.
There are plenty of quality freeware backup applications available designed to make incremental or schedules backups of all your files. Slap in a large capacity drive and install Syncback.

Introduce computing to an extended family member.
We all have at least one family member who still doesn't have a computer at home because they can't justify the cost. You should not hear any excuses when you offer them a free PC.

Rip it apart.
There is no better way to understanding how a PC is put together than by simply taking one apart - and trying to put it back together again. You will be surprised just how easy it is. Let Google be your guide.

Convert it to a Media Centre PC.
A Media Centre PC will change your living room life. I can personally vouch for that. And there are some good freeware Media Centre operating systems available. Or try out a trial version.

Start an over-clocking project.
Do so at your own risk. But if you ever wanted to experiment with over-clocking, try and get that old Celeron 600 up to 1ghz with some extra fans. If you kill it, no loss.

Help find Martians.
The SETI project (Search for Extra Terrestrial Intelligence) works by harnessing the CPU cycles of idle computers to help process data. With a spare computer, you can install the free SETI screen saver which will utilise your CPU power to assist them in, um...finding Martians!

Trade it in.
Some computer dealers will give you a discount on a new PC if you trade in your older one. Just make it look as presentable as possible.

Install Linux.
Why not? It's free and you could learn a lot about how to properly set up a computer that isn't based on Windows.

Feel free to add your suggestions below.