Encyclopedia of Earth. Contains quite detailed information about a wide range of environmentally related topics. It's a growing resource and they are looking for people to contribute.

The forum provides learners an opportunity to become involved in discussing some of the issues relating to the articles.

The site looks like a very credible source of free information, text, images and opportunities to interact that could make it a useful educational tool.

Find out more at http://www.eoearth.org/.