The first great resource to share has to be the Manifesto for Agile Software Development. Please find below other important links and resources ...

Organizations
Agile Alliance
Agile Project Leadership Network (APLN)
Agile University
International Consortium for Agile (ICAgile)
Scrum Alliance
Scrum.org

Tools
CollabNet
JIRA with GreenHopper
Rally
Serena Agile
VersionOne

Interesting Topics
Lean Coffee
The new user story backlog is a map
Visualizing a Large Product Backlog With a Treemap

Books

Articles
Agile Software Construction is Like Neighborhood Construction by Josh Gough
Agile/Lean Documentation: Strategies for Agile Software Development by Ambysoft Inc.
Book Excerpt - Get Ready For Scrum by Ken Swaber and Mike Beedle (PDF)
CIO Playbook For Adopting Scrum (PDF)
CMMI or Agile: Why Not Embrace Both by SEI (PDF)
Controlled Chaos - Living on the Edge (PDF)
Effective Practices for Modeling and Documentation by Ambysoft Inc.
Future of Scrum Parallel Pipelining of Sprints in Complex Projects by Dr. Jeff Sutherland
Kanban Applied to Software Development: from Agile to Lean by Kenji Hiranabe
Introducing An Agile Process to an Organization.pdf (PDF)
Lessons Learned From The First Scrum by Dr. Jeff Sutherland (PDF)
Managing the Work in an Agile Project (PDF)
The Agile Customer’s Toolkit (PDF)
The Agile revolution by conchango (PDF)
The Need for Agile Project Management by Mike Cohn and Ken Schwaber (PDF)
The Scrum Software Development Process for Small Teams by Linda Rising and Norman S. Janoff (PDF)
Scrum Development Process by Ken Schwaber.pdf (PDF)
SCRUM An extension pattern language for hyperproductive software development (PDF)
Reporting Scrum Project Progress to Executive Management through Metrics (PDF)
Risk based selection for agile iterative lifecycle methods by Ronny De Winter
User Experience Activities on Agile Development Projects by Ambysoft Inc.