MIDI Patchbay lets you hook up your various pieces of MIDI software and hardware and pass MIDI data between them, applying assorted filters on the way. Things like key splits, MIDI channel remapping, transposition, etc. are made very simple.
Version 1.0.3: It now has nice icons, thanks to Kevan Staples. It runs native on Intel. MIDI Patchbay and its source code are now under an MIT license.
Mac OS X 10.4 or later