I have tried to put together Microsoft's official twitter accounts in a list below. Please let me know through comments section below if you would lke to add others to the list.
Microsoft - The official Twitter page for Microsoft
Microsoft Windows - The official Twitter page for Microsoft Windows
Microsoft Office - Tweets to help you do more with Office at work, home and school.
Microsoft News - The official Twitter account for Microsoft Corporate Communications
The Windows Blog - Microsoft's official blog for the Windows Operating System.
Microsoft Security - We cover security and privacy efforts for Microsoft - from the inside.
MSDN - The Official Microsoft Twitter account for Developers
Microsoft TechNet - Offical Microsoft TechNet Twitter Feed -- A group feed authored by Microsofties, MVPs and IT Pros.
Windows Developers - The official twitter channel for Windows Developer User Education. MFC, Visual C++, Win32, we got you!
Microsoft Jobs - The official page of Microsoft Recruiting. Find out what it's really like to work here.
Visual Studio - The official account for Microsoft Visual Studio.
MS Web Platform - We are the Microsoft Web Platform Team. Feel free to chat with us, give us feedback, ask us questions and check out our web dev stack.
SQL Server - The official SQL Server: a trusted, productive, and intelligent data platform.
Silverlight - The official Microsoft Silverlight Team Twitter account
Microsoft Exchange - The official handle of the Microsoft Exchange Product Team
Internet Explorer - The official Twitter page for the Windows Internet Explorer team. Conversations, dev tools, and IE news for developers and web enthusiasts.
*New* modern.IE - News and updates from the Internet Explorer Developer Relations team.
Microsoft SharePoint - The SharePoint Product Group
Microsoft Cloud - A source for cloud computing news and the latest on Microsoft cloud solutions like Windows Server Hyper-V, Windows Azure and Office 365.
Windows Azure - The official account for Windows Azure.
AppFabric - Official account for Microsoft AppFabric, which comprises Windows Azure AppFabric and Windows Server AppFabric
SQL Azure - Official tweet of the SQL Azure team.
Virtualization - Microsoft Windows Virtualization team
Microsoft Support - Official account for Microsoft Customer Service & Support; helping you find answers to your Microsoft questions, M-F from 7:30 AM to 5:30 PM PST!
Microsoft Support KB - Official twitter account of the Microsoft Knowledge Base. Bringing you updates on KB articles plus top solutions as a member of Microsoft Help & Support
Microsoft Fix it - Automating solutions to common software problems in an easy, intuitive way that is available when and where you need it.
Windows Live - Official Twitter account of Microsoft Windows Live.
Microsoft Office Live - Tweets from the team at Microsoft Office Live
MSN - The official Twitter page of MSN.com. Stay in the know.
Messenger - News and updates from the Windows Live Messenger team.
Windows Phone - The Official Windows Phone Twitter Channel - keeping you updated M-F 9am-5pm PST with the latest Windows Phone news.
Xbox - Welcome to the official Xbox Twitter account. Jump in!
Bing - Official Twitter account of Microsoft Bing
Microsoft Partner - The official Twitter Channel for the Global Microsoft Partner Network Team.
Microsoft Connect - We're the team that builds connect.microsoft.com, the site that lets you directly engage with Microsoft teams working on the products and services you use.
Security Response - We are the official security response team at Microsoft.
Microsoft SMB - Dedicated to delivering info about how Microsoft’s technologies help small- and mid-sized businesses. Stay tuned for news and upcoming announcements.
Microsoft Research - Microsoft Research is dedicated to conducting both basic and applied research in computer science and software engineering.
Office 365 - Office 365: next gen cloud computing w/ Email, Websites, Document Collaboration, Online Meetings, Presence, IM and Microsoft Office for everyone!
Windows Phone Support - Welcome to the Official Windows Phone 7 Support twitter account. Tweeting M-F 9:00am-5:00pm Pacific
MVP Award Program - Official Twitter account of the Microsoft MVP Award Program
Microsoft Citizenship - Sharing stories and insights about how Microsoft and our partners are working to deliver positive social & economic change around the world.
MSFT Imagine Cup - Microsoft Imagine Cup is a competition that challenges students to use technology to address the world's toughest problems.
Microsoft Press - We publish books & ebooks about Microsoft applications, certifications, tools, technologies, & research. Plus programming best practices.
Microsoft Tech Student - The official Microsoft Twitter account for student technologists in the US. Follow us for great resources, tutorials, free software, career help and more.
Microsoft Surface - Official Microsoft Surface tweets from James Maki of the Surface team. Be sure to check out the Surface Design & Dev Center -- http://msdn.com/windows/surface
Windows Gamer - The former home of Microsoft's official PC gaming community on Twitter.
User Community - The official Microsoft Twitter account for connecting technologists to each other. Follow us for great resources, tutorials, event information and more.
Microsoft Green - The official Twitter account for Microsoft's Environmental Sustainability team.
Microsoft PDC - The definitive developer event focused on the strategy of the Microsoft developer platform.
Microsoft BI - The official Twitter account for all things Microsoft Business Intelligence.
Microsoft Hardware - The official Twitter page for the Microsoft Hardware team.
Microsoft Student - Representing the Student Experience team at Microsoft--we're here to make sure students around the world have good experiences with Microsoft!
Safer Online - Microsoft's official Twitter account for privacy and online safety guidance and information.
Xbox Support - Guinness World Record Holder: Most Responsive Brand on Twitter!
Xbox For Core - Welcome to the Official @XboxForCore Twitter account, for serious gamers. Jump In!
Windows Embedded - The official Twitter account for Microsoft's Windows Embedded Business.
Microsoft Dynamics CRM - Official Microsoft Dynamics CRM twitter feed
Xbox For All - Welcome to the Official @XboxForAll Twitter account. Jump In!
Windows Phone Support - Welcome to the Official Windows Phone 7 Support twitter account.
MS Forefront Team - The official Twitter account for Microsoft's Identity and Security Business - stay informed on the entire Microsoft Forefront Family of products.
Microsoft Robotics - Updates from Microsoft Robotics group and anything cool in the field.
SkyDrive Team - SkyDrive is the one place for the docs, photos and other files that you want everywhere. From Microsoft.
Of course that would be shame to forget Bill Gates' official accout:
Bill Gates - Sharing cool things I'm learning through my foundation work and other interests...
Finally, I would like to add Microsoft Turkey (where I work at) official twitter account: