AtheOS & Syllable Desktop | Forgotten Open-Source Operating Systems

How many operating systems do you know? MacOS, Windows, Linux, FreeBSD, and Android are probably the most well-known. There's many more that are simply too old, require specific hardware, or lack anything interesting about them.
AtheOS & Syllable are a family of OPEN SOURCE operating systems written from scratch that you'll probably be surprised you've never heard of.