Mac: When you're working on a few websites, keeping track of them all gets a little tedious -- especially when they can't all be kept in the same location. Anvil sticks all your projects in the menubar and associates them with a .dev URL for easy access.
To use Anvil, you simply click the icon in your menubar and add any directory containing a static HTML site or Rack app. Although Anvil can't natively handle PHP sites (which is a big downside for me), there is a workaround. Any site you add is given a .dev URL (http://mysite.dev), so you can get to it easily in your browser without dealing with file:// URLs or configuring anything tedious in the command line.
It won't work for every kind of web developer, but if it supports the technologies you use it's a great little bit of added convenience.