The following are useful Yahoo Groups:

....Scrum Development (of course)

....Extreme Programming

....AgileLean - For discussing how to learn, apply and evangelicize Lean-Agile Software Development and/or Scrum, TDD, TOC, Kanban, ... within the context of Lean.   

....Agile Project Management

....Lean Software Development

....Agile Announcements

....AgileBusiness

For Spanish speakers

...Foro Ágiles: for discussion all kind of Agile SW dev  

There are many others.  Please add them.