The WRT54GS, released in the Spring of 2004, uses software to increase wireless speed. It also features improved hardware.
DD-WRT: Based on Sveasoft's Alchemy, Many Features (Latest Version: v23 SP2)
Freifunk: Based on OpenWRT, Accepts Software Installations (Latest Version: 1.0.5)
HyperWRT: Mininal extra features, fast, for most users (Latest Version: 2.1b1)
Linksys: Official Firmware, least features, most stable (Latest Version: 4.20.7)
OpenWRT: Advanced firmware, Linux experience required (Latest Version: White Russian RC5)
Sveasoft: One of the most popular, many features (Latest Version: Alchemy)
TinyPEAP: Based off Sveasoft's Satori, this stable RADIUS server is great for advanced users. (Latest Version: 2.50)
Thibor: Based on HyperWRT with more featuress. (Latest Version: 15c)
Ewrt: Allows users to create hotspots (Latest Version: 0.4.3) - DISCONTINUED

Firmware Recommendations