Probably a modelling pedal (reproduces different amps, speaker cabinets and effects like chorus, reverb, distortion, etc.). You would be looking for something with a midi output. If you check some of the Gear Reviews, you might find some ideas. I originally got a Zoom G1 which had great effects for the price, but no midi. I upgraded to a DigiTech RP250, which actually has better modelling and has midi.

There might be other stuff, but that works for me.