Not sure about interesting, but I wrote a cli tool to apply boolean set operations on lines of text, so e.g. I can get the intersection of the lines in three different files, or all lines piped into stdin that do not appear in a certain file, or merge the lines from multiple files without duplicates.
I made it because this specific task came up repeatedly in my bash scripting but solving it in bash is ugly and really, really slow.
And right now I am working on my own modal text editor which might never be used by anyone else. Just for fun and to see whether some Vim features could be improved or done in other interesting ways. tl;dr so far: Vim does things really damn well, sometimes to a point you don't even think about it because it simply never fails. That said I did manage to finally solve a tiny issue with the keybinding system that always bugged me, resulting in support for smooth text macros as a side effect.