Bootstrap - Simple and flexible HTML, CSS, and JS for popular UI components and interactions
Prepros - Compile Less, Sass, Scss, Stylus, Jade, Coffeescript, Haml and Markdown with live browser refresh.
UIKit - A lightweight and modular front-end framework for developing fast and powerful web interfaces
CodeKit - CodeKit allows you to optimize the performance of your website by automatically and efficiently compiling a variety of popular languages.
Purecss - A set of small, responsive CSS modules that you can use in every web project.
LiveReload - LiveReload 2 proudly presents… The Web Developer Wonderland. (a happy land where browsers don't need a Refresh button). CSS edits and image changes apply live. CoffeeScript, SASS, LESS and others just work.