Jérémy Pinat released Ton: Drum Machine, a free drum app with Ableton Link and Audiobus! This just came out a few minutes ago so there's no demos yet, if one arrives over the weekend I'll plug it into this post.
Ton: Drum Machine iTunes Description:Ton brings a circular step sequencer and an advanced audio engine to your iPad, enabling you to create rich and modulated rhythmic sequences. Its sound-sculpting capabilities are simple yet incredibly deep and will make even the most basic samples sound unique to your style.
• parameter locks: lock different audio values on sequencer steps to modulate your sequence and making it sound natural and organic
• pitch parameter: tune your drums or create basslines
• saturation effect: add a bit of warmth to any sample, or destroy it completely
• multimode resonant filter: attenuate or boost frequencies
• amplitude envelope: shape the overall silhouatte of the sound
• reverb and delay: glue sounds together, from subtle to stellar
• Ableton Link: synchronize with other apps and devices
• Audiobus: send Ton's output to other compatible apps