MIDI Software for Mac OS X
Display MIDI signals, with spying and filtering
Talk with MIDI devices via System Exclusive, and keep a library of SysEx files
Source Code
MIDI Monitor, SysEx Librarian, & SnoizeMIDI
All of the source code for MIDI Monitor and SysEx Librarian. Includes SnoizeMIDI, a general framework for sending, receiving, and processing MIDI.
A Cocoa view to expand/contract an area of a window