"Neptune" was a code name for a Windows release. Windows Neptune was going to be released alongside Windows Odyssey which instead of being a business release like Odyssey, was going to be the first release based on the NT kernel for home users.
A few month(s) after the development of both Windows Neptune and Windows Odyssey the project got actually cancelled and the two development teams of both Windows Neptune and Windows Odyssey were merged together to create a single operating system which was called Windows Whistler that had edition(s) instead of separate releases which were the Professional and Home variant.
Our founder, Kernelpanic, and some friends (including NeoSki) found some references to Windows Neptune and Windows Odyssey, and they discovered that Windows Neptune and Windows Odyssey were actually still alive even after 1999 when they supposedly were merged into the Windows Whistler project.
The "Pictures" category relates to this topic.
Apparently, there was going to be an update called Triton released a few months after the release of Windows Neptune. However, instead of Windows Triton being a service pack for Neptune as originally thought, it was actually going to be a successor to Windows Neptune.
As such, Windows Triton was based on Windows Neptune but of course, it would add many new features and updates.
There are no known build(s) under the code name of "Triton".
Windows Odyssey Plans: 
Windows Neptune Boot Banner: 
Windows Developer Schedule Summary: