Rhapsody

Rhapsody was the code name for an operating system based on OpenStep from NeXT, that eventually evolved into Mac OS X.

Description
Rhapsody was released in Developer Previews, and was designed around the concept of "boxes" -- a "blue box" to emulate the classic Mac OS; and a portable "yellow box" based on OpenStep (with an embedded Java Virtual Machine), runnable on PowerPC and Intel hardware, under Rhapsody, Windows NT, and Linux; and a "red box" to emulate Windows, which was never actively developed.

The "yellow box" eventually became forked into the Aqua environment we know in OS X and iOS, and GNUStep for Linux and Windows. The two environments only have superficial compatibility today.