Microsoft has announced the availability of the the MSBuild tool source code on GitHub as part of their dotnetConf online virtual conference.
Microsoft has added another key tool to their .NET Foundation Open Source library with the release of MSBuild on GitHub. MSBuild is the build system used by Visual Studio, TFS, and Visual Studio Online and is now available for Windows with Linux and OS/X versions coming soon.
The MSBuild sources we’re publishing today are closely aligned with the version we will ship with Visual Studio 2015. You may notice a few differences as this is our first attempt at a standalone build, but you should see those discrepancies reduced over time. And keep in mind that for now, you’ll need to have Visual Studio 2015 installed in order to build the first time. — .NET Framework Blog
You can get the source code on GitHub here, and for more information and announcements check out dotnetConf on Channel 9 going on right now.