Here's a list of music software for OS X:

• Ppd - Free software - a version of MAX, lets you program music in real-time
• GARAGECUBE - Free Software - a port of Miraton for the Mac. Play samples assigned to specific MIDI keys. Used for live performance of electronica or jungle.
• Finale NotePad - Freeware - a notation and playback app for classical sheet music. Doesn't support MIDI input or output or midi files. Playback is very quiet, and you can't alter the mix of two or more instruments (i.e. the organ sound drowns out all others), and you can't adjust the relative volumes. Articulations don't seem to work, despite what the documentation says.
• LilyPond - Free Software - compiles classical sheet music and song scores from text markup like HTML. Not a graphical program. More like a compiler for music. Rosegarden is the front end, but it only works in Linux.
• Ableton Live Lite - Bundled with MIDI keyboards and audio interfaces - Non-crippled version of Ableton Live but missing some features.
• PlayerPro - now freeware but the homepage is defunct. Now archived at SourceForge - Thank God.
• Renoise - Shareware - a new start-of-the-art tracker
• Audacity
• Ardour - Free Software - DAW software for Linux and OS X. Needs X11, JACKOSX, and you must start the JACK server 1st with JackPilot or OJackCtl. You also need to download LADSPA plugins.
• Ecasound - Free software - multitrack audio recorder and processor. command line interface.
• Personal Copy - sound fonts
• CocoaMODX
• Other various trackers exist for OS X, but most don't support editing or MOD file creation.

For PCs & Linux:

• RoseGarden - GNU - only for Linux right now. Needs KDE and JACK
• Studio To Go! - Free Software - Like Knoppix but focused on Audio and Music
• Dynebolic - another multimedia distro
• Seq24 - Free Software - minimal software loop sequencer