LiveReload is a Mac OS X menu bar app for web developers. It monitors changes in folders you specify. When a file is saved, the browser will automatically refresh showing the changes.
LiveReload also comes with various preprocessors baked in (LESS, SASS, Compass, Stylus, CoffeeScript, IcedCoffeeScript, Eco, SLIM, HAML, Jade).
You can use LiveReload without browser extensions by adding a javascript snippet to your page. This plugin does just that.