There's a whole slew of them in the Plugins menu. Rockboy isn't listed in the menu, however; it's a "viewer" plugin, meaning that you select a Game Boy ROM from the file view and it will launch automatically.
However, do NOT ask about downloading ROMs for Rockboy or Pacbox, nor about supplying WADs for Doom. You can supply your own WADs from the original Doom CD or floppies. Game Boy ROMs can be dumped from your own cartridges if you have a cartridge reader.
And because I know this will be asked: No, Rockboy does NOT support Game Boy Advance, Nintendo DS, or any other ROM outside of Game Boy and Game Boy Color.