Google Scholar finds full-text publications, from peer-reviewed journal articles to the grey literature. The most frequently cited publications appear first, which makes it more useful than a regular Google search.
Wikipedia is a multilingual web-based encyclopedia written by volunteers from all around the world, and has become one of the largest reference websites on the internet. Many claims it is as good (or better) than the best commercial encyclopedias.
Eldis, a nice search database , containing more than 22,000 summarized documents from over 4,500 development organizations (free to download).
The Digital Library of the Commons (DLC) is a gateway to the international literature on the commons, and particularly useful for those researching on local forest management.