Back in the 1980s, a certain Apple executive in charge of Macintosh development was forced out of Apple, then went and founded a new computer company. They made a very advanced OS and their own hardware, but stopped producing hardware soon after to focus on software. Their OS was considered by Apple to be used as the basis of Mac OS X.
No, it's not Steve Jobs and NeXT, it was Jean Louis Gassée and Be. Gassée took over as the head of Macintosh development after Jobs left and also developed the Newton MessagePad. He left Apple and founded Be Inc. intending to make a modern operating system and powerful custom hardware to run it. Those became BeOS and the BeBox.
The BeBox aimed to be the ultimate multimedia workstation. It had dual PowerPC CPUs and tons of I/O options including four MIDI ports, multiple sound ports, and the "GeekPort", a 37-pin connector that had power plus digital and analog I/O with a built-in ADC and DAC. Less than 2000 BeBoxes were sold so it's very rare to see one today.
The operating system, BeOS, was developed completely from scratch and had many advanced features for the time. Like NeXTSTEP it was object-oriented, but using C++ rather than NeXT's Objective-C. It had multithreading and supported multiple CPUs at a time when both were uncommon. The filesystem is 64-bit, journaling, and supports indexed metadata, features that wouldn't make their way into mainstream operating systems for years. It is POSIX-compliant so it's easy to port Unix programs to BeOS and it also has a Bash shell, even though it is not based on Unix.
Apple considered purchasing Be Inc. and using BeOS as the basis for OS X but didn't want to pay the price that Gassée was asking. Instead they went with NeXT (paying a lot more), Steve Jobs returned, and the rest is history.
Unfortunately BeOS never really caught on and Be ended up being sold to Palm in 2001.
Let's set up VirtualBox to install BeOS and see what it was like to use!