The Encyclopedia of Earth (website). http://editors.eol.org/eoearth/wiki/Main_Page.
The Encyclopedia of Earth (EoE) is an electronic reference about the Earth, its natural environments, and their interaction with society. The EoE is a free, expert-reviewed collection of content contributed by scholars, professionals, educators, practitioners and other experts who collaborate and review each other’s work. The content is presented in a style intended to be useful to students, educators, scholars, professionals, as well as to the general public. The Encyclopedia of Earth is hosted on editors.eol.org, a collaborative editing facility of the Encyclopedia of Life (EoL). editors.eol.org is intended to provide an accessible, open source option for EoL and affiliated partner projects to maintain text content, with custodial access by the partners to continue their work, manage their communities, and keep their content up to date. (Text from EoE)
The EoE contains information on various topics, such as biodiversity, climate change, pollution, and water, to name a few. It also has pages dedicated to disciplines, such as geography, environmental humanities, physics and chemistry, and environmental law and policy. You can become a regular contributor by requesting an account.