Windows Neptune

Neptune was the codename for an unreleased version of Microsoft Windows which was intended to be the first NT-based version to succeed the Windows 9x series. If released, it would have possibly used the version number 5.50, suggested by the file versions from available binaries, but the operating system reports a kernel version of 5.00. The project, however, was scrapped together with Odyssey, a business-oriented release that would have succeeded Windows 2000, and replaced with Whistler in early 2000. As of 2022, only one build has leaked, but others likely exist.

Major enhancements were planned to ship with Neptune, such as Activity Centers, which were HTML and JavaScript-based applications meant to replace Win32 apps. A new log-on screen was also developed that would later resurface in Whistler. Early builds of Millennium reused files and resources from Neptune; the purpose of these files being to help bridge the gap between NT and the 9x kernel. A lot of these features were reportedly reminiscent of Metro in Windows 8.

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

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.