Be
Someone at work was throwing the CD out and I just had to pick it up. Many OS X new comers don't know this, but Be is an yet another OS that was developed by ex-Apple people. The OS was superior back in its days. I remember feeling awe seeing a video of BeOS running on a PowerPC 603e system performing intensive 3D and multimedia tasks.
Back in the late 90's, Be Inc. was one of the two top runners to be acquired by Apple. In the end Be couldn't join the band. Instead, NeXT (then owned by Mr. Steve Jobs) was bought up by Apple. NEXTSTEP (NeXT's OS) became the foundation for OS X.
Ultimately Be Inc. couldn't survive, sold its assets to Palm Inc. and the OS never saw the light of the day again.