Advanced Visual Beat Editor
aka Microsoft Excel.

One of the nice things about being a one-man team is that I can make my tools as lo-fi as I want. Each “song” in the game is authored in a spreadsheet, like the one above. I use a simple Ruby script to convert the Excel files into comma-separated files, which are then directly parsed by the game (this conversion is automatically done as needed when each file is loaded, using timestamp checks).
As you can see, it’s super minimal. It just supports quarter notes for each hand, and one set of spoken “lyrics” for each player. I’ll extend it to support sixteenth notes, tempo changes, etc. eventually, but this simple format allows me to flush out all my tech and write new songs really fast.