JACK, created by Crudebyte, is a modular environment where you draw connections that can carry audio and MIDI between compatible apps. This is one benefit over Audiobus, which only delivers audio within a set framework. Another bonus is record/playback synchronization between DAWs and sequencer apps. Audio and MIDI data can also be routed from apps to compatible external devices.
JACK is open source (like Audiobus is now), so developers can easily implement the SDK into their app. This is great news, since JACK lacks the 100+ community of apps Audiobus boasts. At this point, only 2 apps support JACK (CMP Grand Piano & MIDI Wrench, both also free and made by Crudebyte). Hopefully that will change soon. With any luck, the competition will drive iOS music to new heights at an even faster rate.