The Agile Alliance is an organization created by some software practitioners to promote Agile Development.
More at: www.AgileAlliance.org 
, www.agilemanifesto.org
-- Mike Beedle
See also: Agile Processes, Agile Software Development
Agile isn't a methodology. Rather, it aims at expressing the existence of a family of methodologies which, according to the methodologists involved, have a commonality of underlying themes and concerns.
Extreme Programming is probably the best-known example of an Agile methodology.
The site only has three pages. It is lovely to look at and easy to read. There is an impressive list of names involved. Here is the first paragraph from their history page
On February 11-13, 2001, at The Lodge at Snowbird ski resort in the Wasatch mountains of Utah, seventeen people met to talk, ski, relax, and try to find common ground-and of course, to eat. What emerged was the Agile Software Development Alliance. Representatives from Extreme Programming, SCRUM, DSDM, Adaptive Software Development, Crystal, Feature-Driven Development, Pragmatic Programming, and others sympathetic to the need for an alternative to documentation driven, heavyweight software development processes convened.
So... How does one go about signing up to join? ;)
Indeed, I would be interested in signing up and use the manifesto in my future dealings with clients saying These are my beliefs and how I like to work, if you disagree then goodbye (you are the weakest link). -- Channing Walton
At www.agilemanifesto.org , click on "Become a Signatory". Choices are: Principles of Agile Software, Become a Signatory, View Signatories, About the Authors, About the Manifesto, Visit the Non-Profit Agile Alliance.
In addition to signing up for the Agile Manifesto, one can also become a member of Agile Alliance, formed to help promote agile processes in the software industry. Go to www.AgileAlliance.com and select the membership section. -- Mary Poppendieck
I think could be useful a list of sites supporting the Agile Alliance, something like Agile User Groups.
You can also look at the User Groups directory on www.agilealliance.com .
-- Marco Abis
See original on c2.com