The Bob application I wrote for OS X in 2006-ish was approved by Apple to be sold in the AppStore. It has a couple bugs here and there, but I thought I'd follow a lean startup mentality and just throw it out there and see if it gets traction. If enough people enjoy it, I'll be more than happy to fix the bugs.
Although the app can be downloaded and starts working as soon as you press the "Start" button. It's probably not super intuitive to make something interesting with it if you haven't been around the MIDI block more than a few times. I thought I would share a brief tutorial on how I made this with NI's Absynth and Bob.
Step 1: Open Absynth
Step 2: File -> Audio and Midi Settings
Step 3: MIDI -> Inputs -> set Absynth 5 Virtual Input Status to "ON"
Step 4: Open Bob
Step 5: Midi Destinations should have Absynth available.
Step 6: Press "Start"
These steps are more or less the same for most MIDI software. Also if you have an external routing box to external synths that should be listed in the drop downs as well.