The FifteenStep MIDI sequencer library that the example uses does not have to be used with USB MIDI. You could use it to output to a standard serial MIDI device, or use the MIDI note data for other purposes. There's an example included with the FifteenStep library that demonstrates using the library with serial MIDI out.
Another possibility is to use the MIDI data to control devices without MIDI capabilities. Below is an example of using an Arduino Uno, NeoPixels, and two MCP4725 DAC breakouts to record MIDI data from a MIDI controller and play it back as contol voltage to a modular synthesizer.
You can find the code used in this example on GitHub.
Thanks to Ladyada & Tony DiCola for the huge memory saving suggestions that greatly increased polyphony in the FifteenStep library, and thanks to Brennen Bearnes for the UI Cheatsheet PDF idea. Also, another big thanks to Marty McGuire and Baltimore Node for the help with laser engraving the Adafruit logo on the sequencer featured in the Make a Sequencer from a Tree video.
A special thanks goes out to my friendly Alaskan Malamute, Biko. He graciously donated his time and beat making skills for the video at the beginning of the guide.