It's not often I'll sing the praises of a Windows app, but sometimes there's that one awesome gem in the crowd that stands out. Today that's Clover.
Clover is in essence a tabbed browsing extension for Windows Explorer. It lets you open multiple Explorer instances in the one window, and it's really quite slick.
The tabs attach at the top of the screen (above the interface elements, in the same style as Chrome). There's addditionally a bookmarks bar which can be disabled, and otherwise the interface is very minimal and gets out of your way. It looks great on Windows XP and 7, although you may get more mileage if you disable the glossy effects in Windows 7 with something like Aero Tuner.
Much of the slickness looks to be owing to the Chromium project, as the tabs and some of the interface elements strongly resemble those in Chrome itself. It fits perfectly in your OS if you use Chrome already, but if you don't it's still an awesome addition to your workflow.
If you use Windows in any capacity you should definitely check it out as it's a feature the OS has been missing for far too long.