Some time ago I discovered a small little helper called Livereload. Its a little ruby gem and chrome/chromium extension that saves you a reload of a page when editing changes. CSS and JS changes are injected even without reloading the page, other changes, like erb changes are refreshed by reloading the page.

From the github description:

LiveReload is a Safari/Chrome extension + a command-line tool that:

Applies CSS and JavaScript file changes without reloading a page. Automatically reloads a page when any other file changes (html, image, server-side script, etc).

Installations is as easy as

sudo gem install rb-inotify livereload

more information is found on the official Github repo

Greg Pollock has done a nice screencast on it: