BeOS was an operating system developed by Be, Inc in the early 1990's. It was ahead of its time in many areas, and some of its more futuristic attributes (i.e. detailed file metadata, 64-bit filesystem) are only now starting to show up in mainstream operating systems. Unfortunately, it never attracted enough consumer interest to remain a viable commercial product. A free version known as BeOS Personal Edition, which can be started from within Windows and Linux is still available, and a dedicated community continues to release patches, software and and driver updates.
BeOS features a POSIX compatibility layer, but is not based on Unix.