Not a personal project, but I've recently decided to start contributing to the Vala compiler. If you're bored and have any interest in compilers, I recommend it. It has a relatively low barrier to entry and the code organization is decent by the standards of 90000 line projects. The barrier to entry is I think a little lower than with rustc, although to be fair I only know a little about rustc.

Other than that—not a coding project, but I have been messing with a novel cement composition. It's stalled due to coronavirus making it hard to get equipment.