Your best bet (only hope, really, of getting the system working again) is to find an OSX installer disc and use the password reset feature on it. Any OSX installer that will boot your system will do.
Perhaps, if you haven't upgraded to Panther yet, now would be a good time/excuse to do so.
(By the way, if you just have an open dialogue asking for a password for whatever reason, but your computer is otherwise set to auto-login, you can just force a restart; you'll at least be able to use it until the system asks for a password again. No way to reset it without either knowing the current password or an install disc, though.)