A: You get two knobs, three buttons and MIDI In/Out on this MIDI Shield. Then connect its output to the microcontroller of your choice. Wow! Control of your synthesizer or effect program.
That's on the DIYDSP store shelves next to the Danger Shield, which approaches controller design differently: It has long sliders and a small LED display instead of MIDI.
BTW, I'm going to save the delightful details for another blog post, but there are some more good-to-know-about items for electronic instrument designers behind that 'shelves' link above :)