DD-WRT is free Linux-based firmware for several wireless routers, most notably the Linksys WRT54G.

Tomato is a small, lean and simple replacement firmware for Linksys' WRT54G/GL/GS, Buffalo WHR-G54S/WHR-HP-G54 and other Broadcom-based routers.

PdaNet turns any android phone as Wireless Modem, USB Tether, Bluetooth DUN, Wireless Router.