ChibiTracker is a portable IT (Impulse Tracker) clone. It is not a 100% clone though, as it adds more features, such as chorus and reverb, stereo samples, an advanced sample editor and more. The user interface also presents some changes, as it can be custom themed and resized.
(commercial) Finale 2012 is a music notation program giving you complete control over every aspect of the printed page while providing full MIDI capabilities and unparalleled audio output.
(commercial) Finale Allegro is a lower cost alternative to Finale 2007 giving educators, performing musicians, composers, arrangers and worship directors all the MIDI and notation tools they need to create publisher-quality scores, while leaving out the detailed engraving options found in Allegro's "parent," Finale.
GarageRemote allows the control of GarageBand's transport functions from any MIDI controller that has programmable buttons.
iVAMP enables any Behringer V-AMP owner who chooses to use OS X, a way of controlling their V-AMP through software via a MIDI connection. All controls operate in real-time so it doesn't matter if you make a control change on the V-AMP itself or within the software. You see and more importantly "hear" the changes immediately.
kinemeplugins is a set of five patches for Apple's Quartz Composer, providing MIDI output capabilities, alternative MIDI input capabilities, external file input, and some NSDocument connectivity.
MidiKeys shows a little keyboard on screen, and you can use the computer keyboard to play MIDI notes.
With MidiPipe you can route, map, filter, convert & input MIDI messages
Note-a-Drill is a utility for learning to recognize notes and play them on a MIDI keyboard. It operates by randomly selecting target notes that you then have to play on your keyboard to advance to the next note.
Timidity++ is a software synthesizer. It can play MIDI files by converting them into PCM waveform data; give it a MIDI data along with digital instrument data files, then it synthesizes them in real-time, and plays. It can not only play sounds, but also can save the generated waveforms into hard disks as various audio file formats.