Via The Windows Experience Blog:
Listed below are official team blogs from the product teams at Microsoft which are categorized by specific product groups.
Windows
Windows Live
Live Search
Windows Server
Microsoft Office
Microsoft Office Servers
General Microsoft Blogs
Microsoft Security Blogs
Mobile and Embedded Blogs
Virtualization at Microsoft
Microsoft SQL and Data Programmability
Microsoft Channels
Microsoft Expression & Silverlight
Microsoft Academic Blogs
Microsoft for the Mac
Gaming, Xbox, and Zune
In the Microsoft Feeds Directory you can also choose to search for a specific blog you are looking for that isn't listed below.