MilkyTracker - MilkyTracker is an open source, multi-platform music application for creating .MOD and .
BassoonTracker - Web-based Amiga music tracker in JavaScript
OpenMPT - OpenMPT is a popular tracker software for Windows.
GoatTracker - This is a crossplatform C64 music editor.
PxTone Collage - A music editing software, capable of using common sounds, custom sounds, and everything in between to make surprisingly good music. Originally in Japanese, but easily fixed.
MuseScore.org - Create, play back and print beautiful sheet music with free and easy to use music notation software MuseScore. For Windows, Mac and Linux.