The Linksys WRT54G (and WRT54GS) make for very nice firewall since you can install any (Linux) software you wish on them. The platform consists of a MIPS processor running at 200MHz, 16M (or 24M) of RAM, and 4M (or 8M) of flash.

Here are some articles:

• The Little Engine That Could by Robert X. Cringely
• Hacking the Linksys WRT54G - a slashdot article

Here are more technical links:

• LinksysWrt54g at SeattleWireless
• WRT54G - LinkSys site
• Linux on the WRT54G