kotlin-ts-mode for EmacsThis is by far my most successful project. It uses the awesome tree-sitter integration in Emacs to provide a nice editing environment for Kotlin.
I play banjo as a hobby (if you're interested, I am currently focusing on the Irish Tenor Banjo, but I also play clawhammer on the American five-string banjo), and was interested in having my sheet music digitally. I turned to LilyPond as a language for transcribing music, but I found some editor support lacking. In particular, the language is not context-free, and because it is so flexible, it's quite difficult to parse. There is an attempt to write a tree-sitter grammar for it, but the grammar is so generic that it is difficult to use as an editor plugin.
This project is an attempt to only support "common" things in LilyPond, and as a result to place some restrictions on how it is to be parsed. As a result, I have been able to write a very easy-to-use parser and Emacs support.
Long before I switched to using Emacs, I used Vim. Back in my university days, I did a bit of work with MIPS and wrote some syntax highlighting support for it. I believe it's been forked by others and made much better in the meantime, but it was one of my first projects.