http://www.theregister.co.uk/2008/02/21/microsoft_goes_open/
http://www.microsoft.com/presspass/presskits/interoperability/default.mspx
http://msdn2.microsoft.com/en-us/library/cc216517.aspx
http://msdn2.microsoft.com/en-us/library/cc216514.aspx
http://msdn2.microsoft.com/en-us/library/cc216513.aspx
http://msdn2.microsoft.com/en-us/library/cc197979.aspx
I don't think I can explain the benefits these actions could have as they're so wide and varied. I'm not mistaking them for pure altruism, however for a Microsoft platform developer, I'm incredibly excited by today's news of opening up API's and providing comprehensive internal documentation. I had a quick flick through some of the documentation available already, on the day of announcement, and it seems to be of high quality and revolving around key windows properties.
Happy times!