QTTabBar is an extension for Windows Explorer that brings tabbed browsing to Microsoft's file browser. It comes with a number of nifty features including a favorites function for launching folders and applications, groupings of folders that can be opened together, tooltip preview functions for a number of file types, and others. It is also highly customizable and hotkey-enabled.
QTTabBar was written by a Japanese developer known as “Quizo” (hence the name QTTabBar) but because of lack of time, he gave the project over to Paul Accisano.
Actually "Quizo" was resumed development.
Open source branch of QTTabBar: http://qttabbar.sourceforge.net/