It depends on the webserver, app, and who programmed/secured it, but sometimes you can alter the URL and get past the login screen.
In other words, if the URL is
http://www.server.com/cgi/login at the login prompt, you can replace or type in
http://www.server.com/cgi/admin or
http://www.server.com/cgi/shared or something like that to bypass the login. Usually it doesn't work, but occasionally it does. NOTE: that is cgi in the URLs; it looks like a q when underlined.
Or try telnetting or ftping to the site. Sometimes they forget to lock that down and you can get in or find info to allow you to login legit.
Kinda lame, I know, but it sometimes works. And no one else had replied yet. I always like to try the simple stuff before bringing out the flame thrower.