I have had this thing for years running White Russian and ran out of disk space and it collapsed. Gave DD-WRT a go and ran into issues configuring the extra USB memory and lost interest. That was at least a year ago.
Now the latest iteration of OpenWrt called Backfire seems compatible and a hellavu lot easier to use, so here I am giving it a crack.
First thing, does the sucker still power up, Nope. Test power supply, its dead as a doornail. 5V 2amp seems common, hunt around the old box and find common as dirt Linksys adapter and Bobs me uncle, we are away.
Go here;l http://wiki.openwrt.org/oldwiki/openwrtdocs/hardware/asus/wl500g
and download http://downloads.openwrt.org/backfire/10.03/brcm-2.4/openwrt-brcm-2.4-squashfs.trx
Password error, after entering:
tftp> get ASUSSPACELINKaaaa /dev/null
Router seems to be bricked, not sure, and the Mac is giving me grief so trying Windows, and this TFTP application
http://www.dd-wrt.com/dd-wrtv2/downloads/others/tornado/Windows-TFTP/tftp2.exe
Fail
Used this
http://support.asus.com/Download.aspx?SLanguage=en&m=WL-500g&p=11&s=2
Router Utility.
After much stumbling around, suspecting it was bricked, finally got the firmware loaded. Now it says please wait for System reboot - taking ages
Failed again. But for some reason now I am able to boot into DDWRT
Run the Asus Utility again, and loaded the standard Asus flash, that worked so now i have a normal router again. According to the openwrt wiki, i should be able to upload the Openwrt flash but that fails too.