Forge is a free command-line toolkit for bootstrapping and developing WordPress themes in a tidy environment using front-end languages like Sass, LESS, and CoffeeScript.
Forge creates a neatly organized source folder with clean and simple scaffolding (base template files, SCSS files, and theme options). The source folder is automatically compiled to your local WordPress install(s) as you save changes and work on your theme. When you are ready to distribute your theme Forge will build it to a folder of your choice or bundle the theme up into an easy to install zip package.
$ gem install forge
$ forge create themename
$ forge link /wordpress/wp-content/themes/my-theme
$ cd themename
$ forge watch
See the user's manual for more information.