Homebrew PCBs

I'm actually having great success making double sided boards with 10 
mil traces - haven't blown a board in a looong time. Its a fair bit 
of work though and I'm starting to feel the need for >2 layers.

I have a couple items made up that I'm probably going to start 
selling commercially soon and so I'm thinking of investing on 
some "real" PCB making equipment. What kinds of things do the board 
houses use for making these multi layer boards? I've done some 
looking around the net but can only find stuff for hobby boards...

Any links or descriptions would be greatly appreciated!