Macwatch is a simple filewatching utility. You give it a list of files to watch and a command to run when those files change and it will invoke the command.
makewatch.py is a companion script that will parse a Makefile for the dependencies of a target and invoke make when that target's dependencies change.
$ macwatch 'command' [files ...]or
$ something_outputting_lines_to_stdout | macwatch 'command'