1. MythTV:

If you haven’t at least heard about MythTV, you’re missing out. For the uninitiated, you should probably try out Knoppmyth. You can spec out hardware a few ways with this, but for run of the mill SDTV, you can get by with older hardware via the use of hardware encoding/decoding. A Hauppauge WinTV PVR-350 can handle the encode and decode operations just fine, even on slow hardware. Look at the EPIA C3 boxes for proof. a 1GHz C3 seems to be about the floating point equivalent of a Pentium III 500. You just need enough CPU power to write to/from the disk, and you’re set. A Hauppauge PVR-350 and a 500GB SATA drive should set you back about $250, and then you’re on your way to your own home-brew DVR!

2. Firewall:

You can use just about any computer you want for a Linux-based firewall. I use an old Pentium II 233 with 128MB of RAM and a 4.3GB hard drive. It works great. You’ll need two network cards in the unit, but those are easy to come by. For a software selection, I highly recommend IPCop. Before I set up my IPCop router, I handily froze 4 variants of consumer grade Cable/DSL routers. I set up the IPCop router at home and I have been trouble free ever since.

3. Photo Frame:

Ok, so this may not be the most electric-bill-friendly use of older hardware, but it works. Install any version of Linux that will run on the box, load up your pictures in a directory, and point your screensaver to that directory. I have a Dell GX110 connected to a 17″ LCD just for this purpose and it never fails to start conversations (usually due to the crazy photos I’ve taken throughout the years.) Set up a Samba share and let your spouse/roommates drag some photos over, too. It can also serve double duty with some other tasks in this list.

4. CNC Controller:

The only real requirement for this is that your computer have 192MB of RAM in it. That’s the minimum requirement to install Xubuntu. The folks at LinuxCNC.org have given the world a great little CNC package for Linux. You need not be limited there, you can do other sorts of Linux-powered work with an old PC. This is a prime example. He’s using DOS to run his BASIC programs, but there are plenty of BASIC interpreters for Linux, too!

5. Classic Game Console:

You can run Linux and ZSNES on just about anything, but you’ll probably want something with at least USB ports on it. Pick up a couple of cheap USB game controllers, grab some NES ROMs, fire up ZSNES and you’ll be playing Street Fighter 2 in no time! Not to mention all of the other emulators you can find. The MAME project will let you run your favorite arcade games, as well. This doubles up well with a MythTV box

6. Home Server:

So you’ve been eyeballing Windows Home Server for a while and want one of those fancy HP boxes with the 4 hard drives? Save yourself the trouble. You can build a Linux box that will do that and more, including BitTorrent downloads, FTP, NFS, Windows Shares, print serving, iSCSI, whatever you want. A Linux home server is really a how-to in itself, and I think bit-tech.net did a fine job of explaining it, so I’ll just leave it to them.

7. Kitchen Computer:

If you’re planning on actually browsing the web with this computer, you will probably want at least a 1GHz processor, just to preserve your sanity. However, a computer in your kitchen can really be handy whenever you’re cooking. You can look up recipes, or just kill time while you’re waiting for the food to finish cooking. Aged laptops work well for this purpose as you can keep more of your precious counter real-estate. Xubuntu, Ubuntu or Fedora Core would be fine for these.

8. Mp3 JukeBox:

If you’re a party kind of person, this would be perfect for you. Have a computer where your party crowd could pick out and queue up the music that they want to hear from your personal collection.
LinuxDevices had a great article on setting up a Music Server, go have a look here.

9. Webcam Feed:

Any computer with a USB port and a network card could qualify for this one. Tired of your neighbor relieving himself in your lawn? Well, put that computer next to your window and follow this guide.

10. Asterisk Home PBX:

Admittedly, this isn’t for the faint of heart. You have to be ready to tinker for hours and hours and hours to want to do this. Still, there’s no easier way to get an Asterisk PBX up and running than with Trixbox. You can go all VoIP, you can go hybrid VoIP/Telco, or even all Telco. Asterisk is very flexible and has many very cool features for the tinkerer at heart. Just don’t forget to take your blood pressure medicine.

Oh, and I still haven’t decided what to use the old PC for. Then again, there’s always the option of tax-deductable donation to a charity!