Agile Manifesto – The cornerstone of agile.
Scrum.org – Produced by Ken Schwaber and Jeff Sutherland after they broke away from Scrum Alliance. The Scrum Guide is an invaluable resource for the basics of Scrum. The site also has a very useful glossary.
Scrum Alliance – A not-for-profit professional membership organization that runs training courses and has an active community.
The London Agile Discussion Group – a fortnightly meetup discussing various topics around agile, Scrum, Kanban.
KanbanDev – a Yahoo forum discussing Kanban.
Lean Kanban University (LKU) – a fantastic resource to learn about Kanban.
Limited WIP Society – a global community embracing Kanban for software development.