During the initial stages, I remember a proposal that involved the shuttle riding piggy-back on a jet that would get it to altitude and speed, detach, and return (a lot like Burt Rutan's SpaceShip One concept)
Piggy-back would have been a terrible idea. You're much better off towing the vehicle if possible. Carrying something piggyback generally means the carrying aircraft has to provide the thrust and most of the lift for both itself and the shuttle vehicle. Towing it means you have to have a vehicle that can provide its own lift (the Shuttle is marginal at best in providing lift - its more of a controlled brick), but all you have to do is provide thrust from the mother vehicle. An empty 747 has gobs of extra thrust to provide...
Kelly Space Technology had a demonstrator of this for the Space Launch Initiative, the program which began in 2000 to develop new reusable launch vehicles, but that program appears to be on the backburner as the insane idea of manned mars missions gets more priority. We would have been better off funding the completion of the X-33, which had its problems, but had cost only about as much as 1 shuttle mission when Bush had it cancelled shortly after taking office (to be fair, it was WELL over budget). The aerospike engines were very promising, and the metallic heat shield would have been much, much more resistant to damage and easier to maintain..