Finger Pro has both of its iOS apps, MoDrum & BassLine on sale for US $1.99:
The two apps are notable for their implementation of MIDI sync over WiFi, demonstrated above.
Here's what they have to say about the latest versions:
With the latest update for MoDrum (v1.1) and BassLine (v3.2), Finger is the first to fully exhaust the opportunities provided by Apple’s CoreMIDI API. Tempo-sync apps on your multitasking-capable device, or get the ultimate band experience by syncing several iOS devices with each other or your favorite DAW!
Seamless audio and MIDI switchover between computers