Microsoft shared ASP.NET MVC source code. Microsoft is embracing open source in a controlled way:
... the license doesn't enable you to redistribute your custom binary version of ASP.NET MVC (we want to avoid having multiple incompatible ASP.NET MVC versions floating around and colliding with each other). But it does enable you to make fixes to the code, rebuild it, and avoid getting blocked by an interim bug you can't work around.
"Classic" open source people may see this as a draconian kind of open source, but it kind of makes sense for companies like Microsoft, that traditionally maintain control over their sources, and for developers who gain the capability to fix problems. I will really be surprised if this model won't start being used by other companies.
And above all, makes room for "truly" open source products like MonoRail :)