I’ve been using this as a replacement for watchr which I use to autorun tests whenever I make code changes. I’ve switched to guard for a number of reasons, chief amongst them being the awesome notification support. You can use growl, libnotify and rb-notifu to notify test pass/fail.
Here’s the Guardfile I’ve been using. This is tailored for Zend Framework 2 so that whenever I modify a php class under a module directory it will run the tests for that particular class. Rapid feedback for the win!