This DIY MIDI Fighter controller is a brilliant example of a project that is a relatively simple build, yet is a polished final product.
The RN-9090 is an extremely well-built MIDI step sequencer that outputs pure MIDI to control external synths.
The Meganome is a DIY MIDI controller created by Patrick Flanagan featuring an array of LED arcade buttons, and continuous controllers.
The Arduinome is an open-source clone of the Monome with easy to obtain parts. The result is similar to the Monome, but with RGB LEDs.