Windows Neptune
Windows Neptune, called Microsoft Neptune in build 5111, was planned to become the successor of the Windows 2000 family as well as the first consumer version of Windows under the NT code.
It was under development in 1999, until it was canceled, along with Odyssey to give way for Whistler, which would be named Windows XP.
XP included the new login screen, help center and user accounts panel originally envisioned for Neptune. However, Activity Centers were eventually dropped.
Builds
Timeline
Early December 1999: "Neptune" ideas sent to testers
December 27, 1999: Neptune (Alpha) build 5111 is released
January 21, 2000: Paul Thurrott reveals that both Neptune and Odyssey are cancelled and will be replaced by Whister.
March 20, 2000: Windows Whistler build 2211 leaks, it looks like Windows Neptune but it isn't.