Tock-OS, Oxid-OS (the safety version of Tock-OS, like freertos/safertos), and Hubris (from Oxide Computers) are the main three RTOSs, Embassy and RTIC are common frameworks (but not classic RTOSs) used for scheduling and resource management on bare metal systems. There are some more niche or non-public ones as well.

For more classic OSs (not real time), Redox is the main one, as well as a lot of research/experimental/teaching ones, including the Blog OS from this guide/submission.