I work with both Arduino and Netduino, so I list all. Pick your poison.
Servo Control: Arduino: Servo control | Abiro
MIDI Remote: Arduino: MIDI Remote | Abiro
MIDI Touch Control: Netduino: Touch the screen… | Abiro
Distance and Motion Sensing: Netduino in distant motion | Abiro
MIDI over Web: MIDI over LAN, kind of… | Abiro
RGB LED Control over Web: Netduino remote control | Abiro
RGB LED Control: The power of abstraction | Abiro
Inspiring ! I like the MIDI - remote
It took approximately 10 hours to make from handdrawn idea to boxed working prototype (including soldering the Freeduino and prototype boards and developing the software). Admittedly that was due to a bet, that I lost (I thought I would make it even faster).
I use the IRRemote library and my own MIDI software.
It's actually quite useful, but the video doesn't give it credit. I'll make a better one some day.
There's a better (relatively speaking) video now.
I especially liked the IR remote project