Oracle effectively started to close source Solaris with 10, I think and then completely closed sourced it with 11. Illumos is definitely a fork, but given that

Oracle effectively started to close source Solaris with 10, I think and then completely closed sourced it with 11. Illumos is definitely a fork, but given that Oracle effectively doesn't work on Solaris anymore it's more or less picked up where Solaris left off, it's probably superior to the commercial Oracle version at this point.