Dropbox is a sweet little tool. I first tried it during a Startup Weekend event; when a teammate suggested we use it to share access to our little project.
Since then, I've found it to be an essential app. And it's particularly useful for working with Drupal. Specifically: for syncing local Drupal sites across multiple systems. And for sharing access to said Drupal sites with other developers.
So here's a quick guide on how to setup a really slick Drupal testing environment. One that will sync across multiple systems and enable quick access for your development partners.