Seattle Wireless is one of the largest wireless networking communities and maintains a large Web site with up-to-date Wi-Fi information: www.seattlewireless.net/index.cgi/LinksysWrt54g.
Linksys Source Code (Linksys, now a division of Cisco Systems, has a support section with GPL source code for its products): www.linksys.com/support/gpl.asp.
The WRT54G Group maintains a mailing list and an on-line support group and is an active discussion forum for the WRT54G: groups.yahoo.com/WRT54G.
Sveasoft, this Swedish company's site is the author's home on the Net and his personal favorite. The Linksys forum is focused primarily on the Linksys WRT54G and offers source code and firmware downloads: www.sveasoft.com/forums.html.
Broadband Reports—this on-line forum has several active discussion groups focused on the major wireless device manufacturers. The Linksys forum is a great source of information and tips: www.broadbandreports.com/forum/equip,16.
OpenWRT is a recent project that promises to offer a hacker's version of the WRT54G source code. At the time of this writing nothing is available, but it's still worth a peek: openwrt.sourceforge.net.