Microsoft's C# and VB Compilers Now Freely Available
infoq.com
infoq.com
But indeed, that's not a very big change, given that they were free before already.
As for free as in freedom, AFAIK it hasn't been made open source.
In this "Microsoft Build Tools" case, they're separating the tools from their runtime support (the .NET framework). You need the framework to run whatever you're building, so it doesn't seem like much is gained by splitting them up.