BeOS is an OS that was being developped by Be Inc.
But unfortunately they went down, but their great OS
didn;t completely die. A german Company yellowTab.
continued the development of this OS, with the new
name Zeta.
( Zeta is under a commercial licence )

There is also an other project, Haiku OS, they
are making an open source clone of the BeOS
( Haiku OS  is under the MIT licence )