The Dynamic Language Runtime, or DLR, is Microsoft's answer to the needs of dynamic language developers. Built on top of the CLR, it provides a common object model and hosting service that allows interoperability between dynamic languages like Python, Ruby, VB, and JavaScript.
The starter kit, called ToyScript, was first presented in Microsoft's Compiler DevLab, a conference for language developers who are targeting the CLR.
The DLR is an open source project and runs on both Microsoft.NET and Mono.