Here are six fairly specific examples that I know of; in each case, the writers have welcomed collaboration.

1. Eric Peterson's formal geometry notes, on using bordism operations and some formal algebraic geometry in homotopy theory. Though the class is complete, the notes are still in progress.
2. Eric Peterson's vector fields on spheres notes, from a class taught by Haynes Miller. These notes are still in progress.
3. Sanath Devalapurkar's notes from two algebraic topology classes taught by Haynes Miller.
4. Andy Neitzke's notes on applications of quantum field theory to geometry, for a class currently being taught at UT Austin (so the notes are still in progress).
5. Andy Neitzke's notes on the moduli space of Higgs bundles, from a class taught a few years ago.
6. My notes on equivariant homotopy theory, from a class taught by Andrew Blumberg. The notes are stable, but not completed.