c64os is a very cool project. it's a modern operating system that pushes the bounds of what's possible on the C64. it's commercial software, available for about $60, and has a full application SDK.

i haven't tried it in a while, but if anyone is interested i have a cross-compiler workspace setup here: https://github.com/woodrowbarlow/c64os-example-app

(the include files probably need to be updated)

i also worked on a python package for bundling/unbundling "car" files (a single-file bundle-archive format used by c64os): https://github.com/woodrowbarlow/c64os-util