This is by far the best tool for the job that I've found: https://github.com/cortesi/modd
In tandem with https://github.com/cortesi/devd I've found it a good setup for web development.
Modd watches file changes and rebuilds, while Devd enables livereload, letting me make changes in my text editor and then see the rendered changes in the browser, side-by-side, in near real-time.
This is for go web development but I'm pretty sure these two tools are language-agnostic.