I usually use modd (with devd), but it's not maintained.
watchexec-cli looks like a good simple replacement for many of my use cases
Paul Smith wrote a simple bash reloader that wraps fswatch
entr is another
notify is a go command using the same library that modd depends on
watchfiles: Simple, modern and high performance file watching and code reload in python. - both a python lib and a cli tool - the cli is very python-oriented