An Apple TV is basically a media player with a nice GUI, a hard disk, network capability, and costs over US$200.
My Ghetto Apple TV is a media player, well, it's a DVD player that plays AVIs through USB port (US$50), with a not so nice GUI, a hard disk (from an old PC), and a NetDisk enclosure (US$19.95).
This is part of a bigger instructable outlining a media network that records off satellite TV and streams media around the house, but I figured some people might just want to watch their AVIs on a TV or through a projector. Hopefully you might already have some (or all) of these components already lying around the house!
Step 1: Find a Media Player With a USB Port
First, you need something to play your media using a USB port. Most modern DVD players will play AVI files and some of them will play AVI files via build-in USB ports. If you aren't sure, just carry a USB flash drive with a test video with you the next time you visit the electronics shop. Often the generic brands can do it even if they don't officially support it. I bought a DSE model G3026. Inside it has a SUNPLUS SPHE828 chipset, which is about as basic as you can get. I've opened dozens of DVD players and lots of them have SUNPLUS chipsets of various flavours. Some have USB, some USB & SD, others don't have either on the outside but support it on the circuit.
I've always been more comfortable with a soldering iron than a credit card, but you may be the opposite....
Step 2: Now Find a Hard Drive That You Can Connect to Your Media Player
Now that you've found a player that works with your favorite video/audio formats over USB, you need somewhere to store it. If you are happy with sneakernet (you don't need the hard drive networked), just find a USB hard drive that the media player will recognise. Some will see most hard drives but the less you pay, the more finicky the player is likely to be.
If your player plays off the USB drive but doesn't recognise your hard drive, try these tricks:
1) Use a hard drive that is 128GB or smaller
2) If bigger than 128gb, try a 128GB partition
3) Format the drive using FAT32
4) Wrap electrical tape around the outside of the USB connector (either side). Sometimes the players think it is a flash drive if it doesn't share an earth (ground) (pic 1)
5) Use a hard drive that has it's own power adapter
So if you could use a spare external hard drive you might have lying around (pic 2).
You might have a drive from an old PC or laptop and you could buy a external enclosure for it (pic 3)
Or if you are really cheap, you skip the external case and just buy the USB to PATA/SATA connector (pic 3) and power it up with an old PC power supply.
Step 3: Now Network Your Media Storage.
Hopefully if you are reading this, your media player plays what you want off the hard disk. You need to find a external drive or enclosure that has a USB connection and can be recognised on your home network. I did this with a Ximeta NetDisk enclosure (pic 1). You connect the hard drive to your media player via USB and to your home network via RJ45 cable. For the NetDisk, the USB has priority over the RJ45 port so if the player is on, it acts like a external USB hard disk, but if the player is off, it acts like a network drive. I've got two media players on our network; KLR (Kids Living Room) and MBR (Master Bedroom) (pic 4).
So now you copy all your media over to your networked hard drive, turn on the media player and start enjoying it on your TV or projector.
I hope that everyone will try different players and through your comments, we can set up a database of the working media player solutions!