CONTEXT INSTITUTE - Publisher of the still very valuable In Context Magazine.  The website has every back issue available.  A favorite issue: Sustainability has an amazing interview with Hunter Lovins
http://www.context.org/

NATURAL CAPITAL SOLUTIONS - This organization was founded by Hunter Lovins and is dedicated to educating senior decision-makers in business, government, and civil society about the principles of sustainability.
http://www.natcapsolutions.org/

AWAKENING EARTH - The website of Duane Elgin who is especially linked to the topic of Voluntary Simplicity.  He literally wrote the book, Voluntary Simplicity - toward a way of life that is outwardly simple, inwardly rich.  The website has a links page that is possibly the best source, most complete and generous website listings for everything to do with sustainability. 
http://www.awakeningearth.org/

CENTER FOR A NEW AMERICAN DREAM - works with individuals, institutions, communities and businesses to encourage them to consume responsibly, conserve natural resources, counter the commercialization of our culture, and promote positive changes in the way goods are produced and consumed.
http://www.newdream.org/

THE SIMPLICITY FORUM - A think tank of academics and authors, activists and artists, educators and entrepreneurs who seek to promote simplicity in our work and practice it in our lives.
http://www.simplicityforum.org/

THE SIMPLE LIVING NETWORK - Since 1996 it has been providing resources, tools, examples and contacts for conscious, simple, healthy and restorative living.
http://www.simpleliving.net/main


Some other great sites well worth a visit - book mark them all!
http://earthfriendlymoving.com/greenbox/ - amazing!  Simple idea and so despartely needed.

http://www.grist.org/ - you might need to laugh from time to time.

http://greenbiz.com/  - sign up for their great newsletter.  Great source for important news in the green world. 

http://www.yesmagazine.org/  - This "replaced" In Context Magazine.  YES!, is published by the Positive Futures Network (PFN) and YES! magazine starts with the belief that we need deep change if we are to avoid the breakdown of society and the natural world.  PFN is an indpendent non-profit organization supporting people's active engagement in creating a just, sustainable, and compassionate world.