Active Directory Domain Services

Active Directory Domain Services Overview
http://technet.microsoft.com/en-us/library/hh831484.aspx
What is Active Directory?
http://www.microsoft.com/en-in/server-cloud/windows-server/active-directory-overview.aspx
Active Directory Services Overview (wiki)
http://social.technet.microsoft.com/wiki/contents/articles/1026.active-directory-services-overview.aspx
Microsoft Active Directory: An Introduction (Microsoft Virtual Academy video series)
http://channel9.msdn.com/Series/IntroToAD
Wiki: Active Directory Domain Services (AD DS) Portal (Fantastic!)
http://social.technet.microsoft.com/wiki/contents/articles/13752.wiki-active-directory-domain-services-ad-ds-portal.aspx
Wiki: Active Directory Features in Different Versions of Windows Server
http://social.technet.microsoft.com/wiki/contents/articles/19037.active-directory-features-in-different-versions-of-windows-server.aspx

AskDS / AskPFEPlat

The AskDS blog remains the most amazing place on the internet to find deep technical answers to all of your AD questions.  Here are links to some of their best articles related to learning Active Directory.  Each of these posts contain links to key Active Directory information that every admin should know.
http://blogs.technet.com/b/askds/archive/2010/07/27/post-graduate-ad-studies.aspx
http://blogs.technet.com/b/askds/archive/2009/01/30/seeing-the-domains-through-the-forest-what-you-need-to-know-to-build-your-career-in-directory-services-technologies.aspx
http://blogs.technet.com/b/askds/archive/2010/06/25/friday-mail-sack-1970-s-conversion-van-edition.aspx (scroll to bottom)
And here are two bonus posts that I couldn't pass up.
http://blogs.technet.com/b/askds/archive/2011/12/08/effective-troubleshooting.aspx
http://blogs.technet.com/b/askds/archive/2011/09/02/accelerating-your-it-career.aspx
First, Do No Harm (DCPROMO checklist, highly recommended)
http://blogs.technet.com/b/askpfeplat/archive/2012/08/06/first-do-no-harm.aspx

White Paper Downloads

Active Directory Domain Services Operations Guide
http://www.microsoft.com/download/en/details.aspx?id=16849
Best Practice Guide for Securing Active Directory Installations
http://www.microsoft.com/download/en/details.aspx?id=16755
Best Practices for Delegating Active Directory Administration
http://www.microsoft.com/download/en/details.aspx?id=21678
Windows Server 2008: Planning for Active Directory Forest Recovery
http://www.microsoft.com/download/en/details.aspx?id=16506
Windows Server 2003 Active Directory Branch Office Guide
http://www.microsoft.com/download/en/details.aspx?id=5838
Read-Only Domain Controller (RODC) Branch Office Guide
http://www.microsoft.com/download/en/details.aspx?id=3608
Planning and Deploying Read-Only Domain Controllers
http://www.microsoft.com/download/en/details.aspx?id=11003
Infrastructure Planning and Design: Active Directory Domain Services
http://www.microsoft.com/download/en/details.aspx?id=732
Active Directory Migration Tool (ADMT) Guide: Migrating and Restructuring Active Directory Domains
http://www.microsoft.com/download/en/details.aspx?id=19188

TechNet Library

How Active Directory Replication Topology Works
http://technet.microsoft.com/en-us/library/cc755994(WS.10).aspx
How Operations Masters Work
http://technet.microsoft.com/en-us/library/cc780487(WS.10).aspx
How the Windows Time Service Works
http://technet.microsoft.com/en-us/library/cc773013(WS.10).aspx
How the Data Store Works
http://technet.microsoft.com/en-us/library/cc772829(WS.10).aspx
Active Directory Maximum Limits - Scalability
http://technet.microsoft.com/en-us/library/active-directory-maximum-limits-scalability(WS.10).aspx
How Domain and Forest Trusts Work
http://technet.microsoft.com/en-us/library/cc773178(v=WS.10).aspx
Capacity Planning for Active Directory Domain Services
http://social.technet.microsoft.com/wiki/contents/articles/14355.capacity-planning-for-active-directory-domain-services.aspx

Understanding FSMOs

Phantoms, tombstones and the infrastructure master
http://support.microsoft.com/kb/248047
FSMO placement and optimization on Active Directory domain controllers
http://support.microsoft.com/kb/223346
Windows 2000 Active Directory FSMO roles
http://support.microsoft.com/kb/197132

Support KBs

Virus scanning recommendations for Enterprise computers that are running currently supported versions of Windows
http://support.microsoft.com/kb/822158
Service overview and network port requirements for the Windows Server system
http://support.microsoft.com/kb/832017
Things to consider when you host Active Directory domain controllers in virtual hosting environments
http://support.microsoft.com/kb/888794

DNS

How DNS Works
http://technet.microsoft.com/en-us/library/cc772774(WS.10).aspx
How DNS Support for Active Directory Works
http://technet.microsoft.com/en-us/library/cc759550(WS.10).aspx
DNS Technical Reference
http://technet.microsoft.com/en-us/library/dd197461(WS.10).aspx
DNS in Small Networks Step-by-Step Guide
http://www.microsoft.com/download/en/details.aspx?id=11156
DNS Best Practices
http://blogs.technet.com/b/askds/archive/2010/07/17/friday-mail-sack-saturday-edition.aspx
Scroll half way down the article.

DNS Scavenging

Managing the aging and scavenging of server data
http://technet.microsoft.com/en-us/library/cc776907(WS.10).aspx
Optimizing your network to keep your DNS squeaky clean
http://blogs.technet.com/b/networking/archive/2009/02/09/optimizing-your-network-to-keep-your-dns-squeaky-clean.aspx
Don't be afraid of DNS Scavenging. Just be patient.
http://blogs.technet.com/b/networking/archive/2008/03/19/don-t-be-afraid-of-dns-scavenging-just-be-patient.aspx
A Complicated Scenario Regarding DNS and the DC Locator SRVs
http://blogs.technet.com/b/ad/archive/2008/08/08/a-complicated-scenario-regarding-dns-and-the-dc-locator-srvs.aspx
How DNS Scavenging and the DHCP Lease Duration Relate
http://blogs.technet.com/b/askpfe/archive/2011/06/03/how-dns-scavenging-and-the-dhcp-lease-duration-relate.aspx

PowerShell

Any administrator worth their salt MUST know PowerShell.  Here are some great links to help you get started.
Windows PowerShell: Learn It Now Before It's an Emergency
http://technet.microsoft.com/en-us/scriptcenter/dd742419.aspx
This five-part video series by Ed Wilson, the Microsoft Scripting Guy, is a fantastic place to start.
Windows PowerShell: Scripting Crash Course
http://technet.microsoft.com/en-us/magazine/hh551144.aspx
List of Free PowerShell eBooks
http://www.hofferle.com/archives/624
GoateePFE – PowerShell for Active Directory
http://blogs.technet.com/b/ashleymcglone
Yes.  I am promoting my own blog as your source for learning how to use PowerShell with Active Directory.

Miscellaneous

Remote Server Administration Tools for Windows 7 with Service Pack 1 (SP1)
http://www.microsoft.com/download/en/details.aspx?displayLang=en&id=7887
This download gives you the admin tools on Windows 7. Enable them from Control Panel - Programs - Turn Windows features on or off - Remote Server Administration Tools.
 
MCITP Certification & Exams
http://www.microsoft.com/learning/en/us/certification/mcitp.aspx#tab2
Use the exam guides linked here for a great list of topics to measure your knowledge.  If you see a term you don't recognize, then study it. 
 
TechNet Virtual Lab: Windows Server 2008 R2: What’s New in Active Directory
https://msevents.microsoft.com/CUI/EventDetail.aspx?EventID=1032437246&culture=en-us
This is a free online lab where you can practice with Windows Server 2008 R2 and Active Directory.  It includes a 2008 R2 domain controller, a Server Core member server, and a Windows 7 client.

Windows Command Reference
http://www.microsoft.com/download/en/details.aspx?id=2632
Wow! This CHM (compressed HTML) help file contains syntax and examples of nearly every Windows command line utility, including Active Directory commands. Every administrator needs a copy of this. Note: After downloading the file be sure to unblock it (Right click the file, Properties, click the Unblock button, OK). Then you will be able to view the contents.