PipSqueak
A joystick and a button from useMIDI. Two CCs for the axes, one for the button.
Circuit Playground
Adafruit's round sensor board, running George's multi-tool MIDI firmware. One sensor per mode.
Midi Fighter
Sixteen arcade buttons, one on/off LED each. Every button is a note; every LED is a note sent back.
Launchpad
An 8×8 grid of RGB pads. The pads are notes; the colours go back as SysEx.
Anything else
Trinkeys, the NeoTrellis, whatever you brought. The generic helper reads it; the Explorer tells you what it sends.