I was thinking about a sort of reverse Tor this morning, so that sites can hide their IP from users, as opposed to the other way around. Trouble is, you want it to work with existing browsers - users aren't going to install a new piece of software or a special browser to access masked domains.
> Does this mean I can access a .onion address from within firefox when I've got the Tor plugin enabled?
Yes.
You will however need to run the Tor software -- Privoxy or Vidalia bundle or... something. I don't think the plugin can do the routing itself.
I haven't used Tor in a while, but there used to be wikis, forums, even shell account servers and search engines available as hidden services. Don't expect anything interesting though.