This page lists the various open source firmware replacements for wireless AP devices such as Linksys WRT54G or Mitsubishi R100 or operating systems for embedded devices such as PC Engines WRAP.
OpenWrt is a Linux distribution for embedded devices, it will install on a wide variety of architectures from wireless APs to x86 - openwrt.org
See also: X-Wrt is a set of packages that enhance the end user experience of OpenWrt - x-wrt.org
DD-WRT is an easy to use firmware replacement for Broadcom based devices such as Linksys WRT54G and Asus-WL500g - www.dd-wrt.com
Ewrt is a firmware replacement specifically for the Linksys WRT54G, maintaining the Linksys look and feel but adding an array of features - www.portless.net
Note: Ewrt project has recently closed and is up for adoption
Freifunk Firmware is a firmware replacement with an emphasis on OLSR. It will run with as little as 2 MB of flash - freifunk.net/wiki/FreifunkFirmwareEnglish
