MidiPipe is way cool. You can basically filter all your MIDI data in various ways: like restrict the velocity to a certain range, reroute channels, split the keyboard between multiple channels, etc.
The main place I wanted to use this is with some of the bass guitar sounds in GarageBand. If you have maximum velocity (or close to it), the played note "slides" up to the actual note, for a kinda cool effect, but I don't want that effect, so I clip the velocity at about 120, and I don't get it
So to play around with this, I used Mighty MIDI to play a MIDI file to MidiPipe Input 1, then added in my pipes (Midi In, then Velocity Modifier, then Midi Out
I didn't even have to set the instruments in SimpleSynth, it just played the programs that Mighty MIDI told it to play.