Microsoft moved two of its development languages "IronPython and IronRuby" as well as Dynamic Language Runtime to Apache open-source license," as company officials acknowledged; "We received customer feedback directly that Apache v2.0 License increasingly became the license of choice for those working on IronPython and IronRuby projects. While Microsoft Public License is a good license, we wanted to support this customer feedback so we decided to make change to Apache v 2 for these projects."
"IronPython and IronRuby are implementations of Python and Ruby programming language targeting .NET Framework and Mono. They're built on top of DLR, a layer of services attuned to dynamic languages that supplements Common Language Runtime (CLR). Until now, these two were available under MS-PL, an OSI-sanctioned open-source license."