OpenMPT - OpenMPT is a popular tracker software for Windows.
Jeskola Buzz - Jeskola Buzz is an all in one digital audio workstation that allows you to edit music with the perfect blending by using modern tools and technology.
SunVox - SunVox is a small, fast and powerful modular synthesizer with pattern based sequencer (tracker).
Tracktion - Tracktion is a multi-platform software for music production, with a modern graphic user interface.
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.
Bosca Ceoil - A free, easy to use tool for creating music.