macWatch allows you to run arbitrary code when a specified file is changed, similar to incron in Linux. To install, make and make install.

For example:

./macWatch important.file "run_testing_suite"

will have run_testing_suite be run every time that important.file is changed.

A common use that I have is, given some script that cds to the right directory and runs make(lets say makeMe this_dir), you can run ./macWatch thisThing.c "makeMe `pwd`". Then, everytime you change the C file, it will re-make, so you will always have an up-to-date executable.