I've been using LiveReload+Guard for this on Rails projects. It'll reload CSS as soon as the file changes on disk without any keypress required. If necessary it'll reload the whole page. It's very handy.

There's a 2.x version that's a standalone OS X app and handles compiling a bunch of things like LESS, Sass, etc. but I haven't used it.

1.x https://github.com/mockko/livereload 2.x http://livereload.com

The ReCSS bookmarklet does a pretty good job of this too. http://david.dojotoolkit.org/recss.html