LiveReload is a Mac-only menu bar app that is quite helpful for web developers. Just tell it to watch a specific folder, and when a file is saved, the browser will automatically refresh showing the change. So no need to switch applications and manually refresh, which is awkward and prone to breaking concentration.
Even better, LiveReload can trigger all the preprocessing to happen first. So if you like to work in SASS, Compass, LESS, Jade, CoffeeScript, Eco, HAML, Slim, or Stylus (or would like to try out working with these languages) LiveReload makes it easy. Essentially, just start making files with the appropriate file extention and LiveReload will compile them down to their native language every time the file is saved.