A few days ago Microsoft dropped the news that they are increasing their openness regarding interoperability. This is great news for the world of software, even though everyone has their own opinions in this matter.
What now have been made public is more than 30.000 pages of documentation of the various protocols used within Microsoft server products (Windows Server 2008, SQL Server 2008, Office 2007, Exchange Server 2007, and Office SharePoint Server 2007). You can find the documentation in the MSDN Library or on the Microsoft Download site.