The official website of Al Gore's An Inconvenient Truth with more than enough information about climate change and what you can do to help prevent it.
http://www.climatecrisis.net/

Fight Global Warming is a great new website from Environmental Defense with plenty of information, ideas, and resources about global climate change.
http://www.fightglobalwarming.com/

Energy Star is a government-backed program helping businesses and individuals protect the environment through superior energy efficiency. It's also a great place to purchase energy efficient appliances.
http://www.energystar.gov/

Track the U.S. government's efforts to promote energy efficiency and renewable energy.
http://www.eere.energy.gov/

The BBC Weather Centre has an entire section of their website devoted to publishing information about global climate change and the impacts it will have on us and the Earth.
http://www.bbc.co.uk/climate/

Earth Day Network has recently launched a Climate Change Campaign to educate consumers, corporations and governments worldwide on the urgent need to take concrete steps on climate change.
http://www.earthday.net/

environmentalresearchweb is a new open-access community website created by IOP Publishing (a not-for-profit publisher, whose work supports the Institute of Physics) to help keep scientists, policymakers and campaigners up to the minute with the latest news and views on environmental topics from around the globe. The site’s coverage is in depth and wide-ranging, from climate change to biodiversity, from renewable energy to pollution, from economics to environmental legislation, and from health issues to sustainability. Thanks to Nichola Downs for recommending this site!
http://environmentalresearchweb.org/

"Yale Global has by far the best collection of articles I've found. They are scientific in spirit, but never full of methodology or jargon. Recently, they have had a two-part 'Planet in Peril' article that is a good overview, also more specific stuff like 'Energy Firms Come to Terms with Climate Change.' As their name suggests, they're really good at what climate change means on a global level." - Kate Letourneau
http://yaleglobal.yale.edu/environment/recent.jsp

"Union of Concerned Scientists (ucsusa.org) gets their climate change concern on in a way that is fiesty, yet credible. Often has links to good articles." - Kate Letourneau
http://www.ucsusa.org/

RealClimate.org is an excellent blog about climate change, written by climate scientists. It's a great way to keep up to date about news and discussions coming out of the world of climate science. Thanks to Arvind Nair for recommending this site!
http://www.realclimate.org/

Wikipedia offers one of the most comprehensive yet readable scientific explanations of global warming and climate change, complete with charts and graphs.
http://en.wikipedia.org/wiki/Global_warming
http://en.wikipedia.org/wiki/Climate_change