Here are six fairly specific examples that I know of; in each case, the writers have welcomed collaboration.
- 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.
- Eric Peterson's vector fields on spheres notes, from a class taught by Haynes Miller. These notes are still in progress.
- Sanath Devalapurkar's notes from two algebraic topology classes taught by Haynes Miller.
- 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).
- Andy Neitzke's notes on the moduli space of Higgs bundles, from a class taught a few years ago.
- My notes on equivariant homotopy theory, from a class taught by Andrew Blumberg. The notes are stable, but not completed.