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SEQ541 is a MIDI pattern sequencer for OS X. The program is extremely flexible, all it requires is that you have an external MIDI device generating a clock and it will output notes in time to a pattern sequence you specify. Why is SEQ541 for you?
The author: "I wrote this program because I wanted a way to control my Roland SH-32 synthesizer (which does not have a built-in sequencer) in time with a Korg ER-1 Rhythm synthesizer (which does have a built-in sequencer). It works pretty damn well for this purpose, especially live, where I would rather be playing pre-programmed melodies and basslines and tweak the knobs in real-time."
If you have a MIDI synth with a built-in sequencer, you can use SEQ541 to visually edit sequences for the device, then play then back into the device while it's recording. If you're a software synth person, SEQ541 works really well for controlling software synths like Native Instruments Absynth or Waldorf A-1 in time with your external MIDI hardware.