orDrumbox is a software drum machine on a desktop application. A fully-featured drum machine and audio sequencer designed to a creative pattern-based way of drum programming. You can compose music and beats with this new generation of sound tracker.
MU.LAB is a software application that transforms your computer into a real music studio. It does not overwhelm you with a complex feature set, in which you can get lost. On the contrary, MU.LAB is a user-friendly yet rock-solid musical tool designed to create, fine-tune and play Your Music.
Glitch Sequencer is a free cellular automaton based MIDI sequencer for the PC and Mac. It offers an interesting style of composition based on evolving patterns and is great for fast, fluid experimentation.
BankStep is a software emulation of hardware bank sequencers found in the big analog synthesizers, that produce control for the sound but not the sound itself. Instead of analog control voltages BankStep produces MIDI (and can make standard MIDI music files).
Otherwise, BankStep functionally emulates a big $50,000 bank of hardware sequencers and supporting modules just as it might sit on top of a big Moog analog synthesizer - or your soft synth, MIDI module or sound card.