The most common instance is of course in keyboards, which seems somewhat natural since it's an interface that was an abstraction far before the digital age -- having been used to such disparate instruments as pianos and pipe organs.<p>For stringed instruments, what's been common for some time is MIDI pickups on either guitars or (less frequently) on basses.  Then they can either be used to drive a synth or you can take the normal analog out.<p>MIDI drum pads or MIDI triggers on acoustic drums are also common.<p>I think much of the appeal for musicians with many years of playing behind them is that they don't have to master a new interface for music input.  I've played bass for 15 years; I can do a lot of phrasing intuitively that's much more tedious to enter by hand into a sequencer or to play on an electronic-specialized instrument that I don't have mastery of.  Granted, I don't actually have a MIDI bass pickup, but I've been tempted.<p>In Squarepusher's music, for example, you can often hear in his synth lines that they were played on an electric bass (which he's exceptional at) because the phrasing is much more idiomatic to an electric bass than to a keyboard.