Microsoft has released the source code for its Common Compiler Infrastructure (CCI) under one of its OSI-approved open-source licenses.
As of this week, Microsoft is making the CCI bits available for download under the MS Public License (Ms-PL).
Microsoft officials describe CCI as "a set of components (libraries) that provide some of the functionality that compilers and related programming tools tend to have in common." The components provide the ability to read, write and manipulate Microsoft Common Language Runtime (CLR) assemblies and debut files.
Microsoft has used CCI internally to develop various programming languages and other tools, including FxCop and Spec#.
Earlier this month, Microsoft released its ASP.Net Model View Controller (MVC) code under the Ms-PL open-source license, as well.
Update: Speaking of new open-sourced tools on CodePlex, there's a new Facebook API client (in alpha form) that sounds interesting. The new tool, developed by Daniel Schaffer, a social engineer for LiveCash (and self-avowed .Net lover), is meant to "greatly facilitate the development of 3rd party Facebook applications for developers and development shops already familiar with the Microsoft .NET technology stack." It's available under the Ms-PL license. (Thanks to Softie Scott Hanselman -- who has compiled a useful list of a number of Web 2.0 tools that may be of interest to .Net developers -- for the link.)