The early 2000s really took a toll on alternative commercial OS offerings. OS/2, QNX (on the desktop), BeOS, and others made that time interesting. There were even a lot of mobile OS options (Palm, early Linux handhelds, Symbian, Windows PocketPC/Mobile).
BeOS was always my favorite though. It’s amazing what Haiku has done, but sad that BeOS was shelved 20 years ago.