WinFS (Windows File System), which was originally supposed to be included (and be a core feature of) the upcoming Windows Vista (previously Longhorn), has entered "Beta 1" and is available to MSDN subscribers. WinFS is the new file system that will supposedly revolutionise the way we store, manage and retrieve files. Beta 1 provides support for Windows XP, leading many to think that the release version may provide such support as well.
Read Tom Rizzo's blog for full details.
Original sources: Microsoft Watch, ThreadWatch and Scobleizer