Hydrogen

Screen dump: 
Hydrogens interface showing the pattern editor, pattern sequencer and mixer

Hydrogen is a pattern-based drum sequencer. It allows you to create patterns and then sequence them into songs. Drum instruments are tweakable and hydrogen allows for up to 16 layers of velocity per drum instrument. Furthermore, Hydrogen includes a mixer and has support for LADSPA plugins. It can be used in sync with other programs. You can also export your songs on a per instrument basis if you prefer to process them in another program. Hydrogen works with both ALSA and JACK MIDI, so it can be controlled via an electronic drum kit or pads if you wish.