Local web testing on Mac, Windows and mobile devices
If you do web development you need to test across multiple devices, browsers and operating systems. My primary development machine is a Mac however I also run Windows 7 via Bootcamp/Parallels for browser testing locally (and for the occasional .Net based project that I take on). Now that mobile has become such a factor in the web development process it is also critical to test across devices. Being able to quickly test web sites and web apps without having to push files to a staging server is a huge time saver.