Microsoft has released Oxite – an open source, standards compliant and highly extensible content management platform (via Amit). Also, Oxite is being marketed as a blogging engine. However, the language used seems to put it more as a CMS platform targeted towards developers than a blogging engine for the end users. This is good news for the open source community.
One thing I would be interested in, especially because it is coming from Microsoft, is whether it will be tied to rest of the Microsoft, like the MS SQL Server. The open source market for frameworks and platforms is growing to be portable across multiple servers, databases and operating systems. It will be interesting to see whether the open source community will get a chance to enable this. And if we will see reflections of this in the Mono world.