Probably the most immediate winner is Unity. They've been stuck on an ancient version of Mono because they did not want to license tech from Xamarin. They can now have MS derived runtimes free.
Unity has been developing their own runtime for a while now to get away from the mono licensing costs of using a new version. I'll be interested to see how they react to Microsoft open-sourcing a lot of stuff they might have been able to use.
Wow, didn't even think about that. Huge step for the Unity project.