Cantor is no longer free. It is a Microtonal Guitarist’s synth, designed for extreme playability on the iPhone and iPad. It is the successor to Geo Synthesizer and Mugician. When Cantor is free, Mugician is for sale and vice versa.
It does send a specialized MIDI output (requires a multi-timbral synth that can handle independent channels is all but necessary, such as SampleWiz or ThumbJam in OMNI mode, or Arctic Synth). The internal synth is driven by the actual MIDI output, because external MIDI synths are the main point of this project rather than the internal audio engine. It has a basic looper/delay combination, and the loop can be copied out as a sample to the general pasteboard for apps such as GarageBand.
It is very expressive in terms of per-finger filtering (channel pressure tracks finger-area, CC11 tracks vertical position).
Cantor is available for $2.99