I just spotted this rather useful looking website called Encyclopedia of Earth. It is pretty much what it says on the box and delivers quite detailed information about a wide range of environmentally related topics. It's a growing resource and they are looking for people to contribute.
There is a forum too so students can get involved in discussing some of the issues relating to the articles.
The site looks like a very credible source of free information. A lot of the articles tend to have a US focus, but there's still quite a few that don't.
Hope you enjoy it.