The Intellivision Music Synthesizer plays Breakout! The keys act as absolute position inputs, so if you want to go the rightmost position on the screen, then you press the rightmost key on the synthesizer. The black keys move the paddle a little bit higher up the screen, and the white keys move the paddle back down to their original height. The graphic along the bottom of the screen shows which key (or pair of keys) you are pressing at any moment.
I'd call this a "work in progress" but it is more of a quick demo that I wanted to share before heading to PRGE. The .rom is available to download, but be warned that I haven't tried it on real Intellivision yet and the footage is done with jzintv.
I'll also attach a .rom that you can use to test an Intellivision Music Synthesizer to see if it has any issues with the keys. The source code for both projects is mostly written in IntyBASIC, and the source code for the testing program is attached and is also available here. Thank you to @intvnut for posting the basis for this project (including IntyBASIC callbacks) in this post.