announced this morning that the source code for its .NET framework libraries is being opened to the world, allowing outside developers to see inside when developing their own software in the .NET framework.Microsoft
It's hard to say what the incentive was for this move, it could be that .NET adoption in the developer community has been so small that a drastic step was needed or this could be a strategy to prepare for a big push of SilverLight, Microsoft's new runtime for Rich Internet Applications. The .NET framework is also at the center of Vista, which so far no one has wanted to use at length.
We're still chewing on the significance of this announcement; hopefully some .NET developers can chime in in comments. Update: Unsurprisingly, Mary Jo Foley's All About Microsoft blog has excellent coverage of this announcement. See discussion as well via Techmeme.