EH-Net

Im interested in buying a hard firewall and would like to know what would be the best one for a reasonable price.

Price:  Price of a cheap box, either free to 500$

OS:  FreeBSD firewall.

Works pretty darn well, and won't bust your wallet.  Others here may have better ideas, but that's my 'firewall on a shoe string'. 

For power, simplicity, and price I suggest Monowall or PFSense. Can be installed on an old system like a pentium 200, which you can probably get for noting.

Seconded, the m0n0wall and PFsense family usually provided everything you need. Just be wary if you're running this in a heavy usage environment as it's performance can start to suffer at 20Mb+ depending on ruleset optimisation and utilised features.

Can't think of anything better for small environments though.

Ebay ....
http://search.ebay.com/netscreen-5xp_W0QQfnuZ0QQfsooZ2QQfsopZ32QQrprZ8QQxpufuZx

Agree. Smaller netscreens and pix's are also good options. Provided you can find them for a good price. Also be aware, you may or may not be able to get software updates for a second hand device, vendor policies differ.

I have been using IPCop for a while now.  (www.ipcop.org).   It works pretty well, although development is a bit slow.   Mine runs on a small-factor compaq box which I bought on ebay for $30.    One nice feature is integrated Snort.