Yesterday, Microsoft made MSBuild open source and available on GitHub. For the uninitiated, MSBuild is an automation build tool that was, until yesterday, part of the proprietary .NET Framework. It is the backend engine behind Visual Studio.
MSBuild is now officially contributed to the .NET Foundation.
Now that MSBuild is open-source, you can call msbuild.exe and use it in environments where Visual Studio is not installed.
MSBuild is not without an aura of controversy, and has in the past been strongly criticized by some developers. (See, for instance, comments on Hacker News regarding the announcement.)
Controversy or no, it is an interesting move in the ongoing march of proprietary tools going open-source, and the latest in a string of open-sourcings in recent months. See also the Open Data Platform.
See the official announcement about MSBuild on MSDN blogs here.