Supplanted by the CodeKit app on the very same site: http://incident57.com/codekit/
(and it also supports Sass, Compass, Haml, CoffeeScript)
(and it also supports Sass, Compass, Haml, CoffeeScript)
My only complaint is that it only works locally, meaning there's no way to fit it into my workflow on projects that involve testing on remote servers. (Admittedly, watching for filesystem changes via SSH/FTP is non-trivial.)