You want to look up 'chording keyboards' - They're one of the few things from Engelbart's 'Mother Of All Demos' that's not now a standard part of computing.
Unfortunately, designers these days seem to hate modal interfaces because they detract from "walkup usability", so I doubt we'll see these making a comeback.
Then again, maybe Apple's multi-touch gestures are close enough to chording that they'll be rediscovered and become cool again.
