Pick up a Brownell's catalog and scan through to see which guns could be built from parts, from a bare reciever on up.
The heavyweight champeens seem to be:
Semiauto Pistols: 1911. You can't really build a Ruger MkII or Glock from a bare third-party frame, but everything else under the sun is available.
Revolvers: None that can truly be built from a bare frame, but there's oodles of stuff for S&W double actions and Ruger single actions.
Rifles: AR15, Rem 700, Mauser, 10/22.
Shotguns: Again, none that could be built from the ground up, but the Remington 870 has far and away the most stuff available for it.