According to Stephan, the only missing feature in MuteKey is, that you can't control it via MIDI. And, there was no auto-updater (like Sparkle). These things shall change now.

I proudly, well sort of, present to you: MuteKey 1.1. Including Sparkle for updates and a new MIDI control interface. Get yourself a copy below!

Some notes on MIDI

The MIDI-support is pretty rudimentarily. MuteKey responds to the note E4 on every channel. Every time you press this key, MuteKey mutes the current input, if you then release the key, MuteKey will unmute the input.