This simple MIDI effect app for Audiobus 3 allows you to transpose notes. You can shift by octaves and half-tones. This is useful if you want play an external keyboard in a different tone. You can also use it to create harmonies, i.e. create a second voice in an interval. Since this app is so essential, you can get it for free.

+ Shift notes in half-tone steps
+ Shift notes in octave steps
+ Use multiple instances of this effect in Audiobus

NOTE: This app requires Audiobus 3. Insert it into one or more MIDI effect ports to affect the data going through the respective pipelines.