http://www.matrixsynth.com/2012/03/boss-mpd-4-midi-controller-pad-trigger.html

Convenient sized MIDI pad controller from Boss, ideal for triggering other midi equipment. Simple MIDI channel, MIDI note, and sensitivity controls for fine tuning the settings. Built like a metal tank, with a fantastically responsive pad.

Ideal for drummers wanting to add digital drum sounds to an acoustic kit, but can also be used to trigger samples synth stabs, etc.-- many uses.

You can use a footswitch (not included) with it also to trigger 2 different sounds from the same unit.

You can also connect 3x Boss BP-1 controller pads via the sockets on the rear for more timbres (BP-1s not included).