Climate Access: Their newsletter always comes packed with valuable articles and their webinars bring the top players in climate change communications together. I learned a lot in their previous webinar on communicating risk. The information can be a difficult to access though (ironically) because their registration process is a bit involved. Worth signing up for if you can though. Their UK counterpark, Talking Climate, is also a useful site.
Both Columbia University (Center for Research on Environmental Decisions - CRED) and Yale University (Yale Forum on Climate Change and the Media) have centers devoted to the psychology of addressing environmental issues. CRED's report on climate change communication is a great resource as is Yale's Knowledge of Climate Change Across Six Americas.
International Environmental Communication Association: Just formed last year. Their previous conferences have all been held in the US, but one in 2013 is planned for Sweden.
EcoAmerica: Their tag line is "start with people" and they've got a great archive of publications with that in mind including the revealing annual American Climate and Enviromental Values Survey.
I'll close with a great slideshow produced by my friends (and fellow Tar Sands Action arestees) Sieren and Yiming on tar sands impacts. What makes the presentation so great is its balance between the science of climate change, striking imagery and personal stories. The biggest take-home lesson I've learned from all the sources above is that climate change communications have relied too heavily on data in the past, that stories and pictures are often more convincing than science. This presentation is a great illustration of what works and Yiming told me it was well-received in her own recent visit to Congress. Feel free to share!