DarrinEden has a few of these to donate to hackers taking on projects like [http://www.seattlewireless.net/index.cgi/LinksysWrt54g flashing a tweaked Linux].
One of the more talked about custom firmwares looks to the [ftp://ftp.sveasoft.com/pub/ one from Sveasoft]. It touts SSH, telnet, TX power setting in the web interface, the ability to do client mode and tons more goodies. The are looking to fold in [http://nocat.net/~rob/wrt54g/ Rob Flickenger's NoCat Splash] and Wondershaper soon.
Getting kismet_drone to run on the WRT54G (July 2004): http://toys.lerdorf.com/archives/20_Kismet_on_the_Linksys_WRT54G.html