Haiku Project
BeOS (the OS Haiku is a reimplemntation of) has an interesting history. It was an OS that has a fully async C++ API (very novel for it's time and even now). The fact that it was async made the OS much more responsive and had a better CPU utilization.BeOS was another company that Apple considered to purchase but they ended up buying NeXT instead. IIRC they ended up going with NeXT because BeOS didn't have networking back then.