Note: this post is from 2007, outbound links may be broken.
arcDev Noise Industries releases DH_MidiMunger
MidiMunger allows you to remap / modify / generate midi messages (note on/off, controller, pitchbend, key/channel aftertouch, program change) in realtime.
It consists of four modules – each is identical except that the last cannot pass its output to anything but the master midi output, whereas the others can optionally pass their output to the next module in line.
MidiMunger comes with a help file explaining how things work, and a bunch of presets are included so you can get an idea how to route things. You will need a host that will route incoming MIDI > MidiMunger > VSTi (or midi output).
Visit arcDev Noise Industries for more information and a link to download DH_MidiMunger.