Longhorn Reloaded - what Vista might have been
Vista was instead built from the Windows Server 2003 kernel. But the Joejoe Group has gone back to the earlier build 4074 of Longhorn and continued development from there. The group has now released the unofficial (and unsupported by Microsoft) Longhorn Reloaded M1 Technical Refresh. The latest version can be installed on a partitioned hard drive along with your native operating system.
Longhorn Reloaded features temporarily abandoned by Microsoft like WinFS, and features that eventually found their way into Vista like Aero. So far, Microsoft hasn't sent any cease and desists to the Joejoe group, but the Longhorn Reloaded is almost certainly a violation of Microsoft's terms of service, so it'll be interesting to see how the project develops both from a technical and a legal standpoint. Right now, you can download the operating system for free. Don't expect all of your XP or Vista drivers to work though.