RISC-V International
...
NuttX RTOS has a weaker sense of maintainership than many open-source projects. However, the primary contributors are:
FreeBSD has a weaker sense of maintainership than many open-source projects. However, the primary contributors are:
...
Illumos has been ported to the Allwinner D1: https://github.com/n-hys/illumos-gate/wiki/Allwinner-D1-Nezha
...
Since then a range of Distributions have RISC-V ports. E.g.:
...
There are multiple RISC-V ports of xv6, including one for the xv6 (https://pdos.csail.mit.edu/6.828/2022/xv6.html).
...