OpenMPT - OpenMPT is a popular tracker software for Windows.
OnlineSequencer.net - OnlineSequencer.net is an online music sequencer that allows users to create beats and music tracks.
SunVox - SunVox is a small, fast and powerful modular synthesizer with pattern based sequencer (tracker).
AudioSauna - AudioSauna is a full-featured audio workstation for making music online.
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.
Jummbox - A modified Beepbox that have more stuff then Beepbox. It can be used just like Beepbox. The features of Jummbox is the same of Beepbox's features, but Jummbox have more stuff to offer.