LiveReload is the essential Web-developer tool: when you save a file, LiveReload pre-processes it as needed and refreshes the browser automatically. Even cooler, when you change a CSS file or an image, the browser is updated instantly without reloading the page.
CoffeeScript/IcedCoffeeScript, Compass/SASS, LESS, Stylus, HAML, Jade, SLIM, Eco are supported out of the box. Works great with many popular editors: Espresso, Coda, TextMate, Sublime Text, BBEdit, SubEthaEdit.
Note: Now requires OS X 10.7 or later.