Windows Neptune

Neptune (sometimes stylized as 'NepTune) is a codename for a canceled version of Microsoft Windows which would have been the first consumer version based on Windows NT, at first succeeding Windows 98 and later Windows Me. Major user experience enhancements were planned to ship with Neptune. This included Activity Centers, a new task-based user interface that involved a group of applications, each focusing on a different area of functionality, that grouped common tasks together in a common web-based interface. The management of multiple users was also improved in Neptune with the introduction of a new login screen and user manager. The new interfaces were primarily using web technology, often using the new Mars framework.

Another key point of the Neptune project was to experiment with new experiences that didn't require the user to manually save files. Some of this work is visible in the available build, which enables hibernation by default and requires the user to take extra effort to shut down the computer regularly. A fast startup feature was also introduced, which saves the state immediately after boot and then restores it on further boots.

The project was ultimately scrapped in early 2000, and the team merged with another team working on Odyssey, a business-oriented successor of Windows 2000, to work on what would later become Windows XP. Many of the project objectives did, however, end up being inspiring some of the new features of the versions that followed, with Windows XP introducing the newer Start menu design as well as certain bits of the task-based interface. The Metro interface introduced with Windows 8 also shared a lot of common goals with the Neptune project, including the Activity Centers' focus on typography or the introduction of Hybrid Boot to reduce boot times.

Triton
A minor update to Neptune called "Triton" was also mentioned in internal Microsoft documents from 1998. Triton was planned to be released in March 2001, 12 months after the initial release of Neptune, which would also receive five Service Pack updates during its development.

List of known builds
Although only build 5111.1 is currently available, there is some evidence for other builds as well, mostly from file versions in various Windows Me builds.