Nowadays more and more work, communication and collaboration involves multiple external parties. This can involve by example employees, customers, partners, suppliers, cloud providers/platforms/applications.

This means it is becoming increasingly important to have proper authentication and authorization methods in place for single sign on (SSO) so users can be more productive. Besides the ease-of-use It can also lead to better security.

Microsoft’s Active Directory Federation Services (ADFS) will make this possible. For more information on ADFS, here are some resources. Keep in mind though that while some information may be outdated, it will give you a broad idea of the concept and the inner workings. The current version of ADFS in Windows Server 2012 is 2.1 , while Windows Server 2008 uses 2.0

• Active Directory Federation Services (TechNet)
• Active Directory Federation Services,Part 1: How Do They Really Work?
• Active Directory Federation Services,Part 2: Building Federated Identity Solutions
• Active Directory Federation Services 2.0 solution guide (including ForeFront UAG)
• Active Directory Federation Services 2.0 Deep Dive: Deploying a Highly Available Infrastructure
• AD FS 2.0 Step-by-Step and How To Guides
• AD FS with Office 365 Step by Step Install Guide
• ADFS 2.0 Setup Guide
• A Guide to Claims-Based Identity and Access Control (2nd Edition)
• Choosing Between Forefront TMG or Forefront UAG for Publishing Scenarios
• Cloud Security: The Practical Meaning of Security, Identity and Access Revealed!
• Complete walkthrough: Setting up a ADFS 2.0 Server on Windows Azure IaaS and Configuring it as an Identity Provider in Windows Azure ACS
• Demystified Series: Active Directory Federation Services (AD FS) Part 1
• Demystified Series: Active Directory Federation Services (AD FS) Part 2
• Deploying and Configuring ADFS 2.0
• Federated Identity in the Cloud [NZ 2010: SEC402]
• Federation and Federated Identity: Part 1 Active Directory Federation Services – How Do they Really Work?
• Federation and Federated Identity: Part 2 Building Federated Identity Solutions with Forefront Unified Access Gateway and ADFS v2
• Forefront UAG and ADFS: Better together
• Managing Identity in the Cloud with ADFS and Windows Azure – TechDays 2012
• MS Innovations Blog posts tagged with adfs
• Office 365 ADFS virtual lab
• Office 365 for SMB Jump Start (03) | Office 365 DirSync, Single Sign-On & ADFS
• Office 365 SSO: A Simplified Installation Guide
• Plan for and deploy AD FS for use with single sign-on
• Setting up Single Sign on with Office 365 using ADFS 2.0
• The Access Onion – Peeling away at Access Management, Identity Federation, PKI and SSO issues
• What’s new in Forefront UAG Service Pack 2 (also ADFS 2.0 changes)
• Windows Authentication, ADFS and the Access Control Service 
• Windows Server 2008 (R2) Active Directory Federation Services (TechNet)

PS: Microsoft is moving more and more towards claims based authentication. Examples include Windows Server 2012 Dynamic Access Control and also SharePoint 2013 that has switched to claims based authentication by default now.

If you have some other resources that might be useful, please let me know so I can add them as well.