The National Science Digital Library - http://nsdl.org - An online library of resources for science, technology, engineering, and mathematics education and research from the National Science Foundation.
Exploratorium Digital Library- http://www.exploratorium.edu/library - The collectioons includes articles, videos, images, exhibits and activities related to science.
Scientific American - http://www.sciam.com/ - You can search the Scientific American website and find articles from past issues. Good for science research projects.
Science Books Online - http://www.sciencebooksonline.info/ - Links to free books online. Use the menu of topics on the left. Don't get confused and click on the links in the Google Ads. They are free!
Insect Images : The Source for Entomology Photos http://www.insectimages.org
"Thousands of (mostly) public domain photographs of insects and related organisms and thousands of photographs on related entomology and forestry subjects (plants, animals, plant diseases, etc.). Reach these through Statistics, Forestry images, Forest Pests, and IPM images links."
Biology Online - http://www.biology-online.org/ - "Answers to All Your Biology Questions."
The Encyclopedia of Life - http://www.eol.org/ - More than 150,000 pages of “expert-verified” text and images. A search for the domestic cat returned Felis catus Linnaeus,1758, accompanied by photos and a description of this favorite pet. The references included a book on cats in Roman civilization and Guinness World Records which lists the oldest living cat. EOL is an ambitious project and anyone may add text, images, videos, etc. which are authenticated by expert curators
General Research Links
http://www.openthesis.org/ - OpenThesis is a free repository of theses, dissertations, and other academic documents, coupled with powerful search, organization and collaboration tools. Theses and dissertations are an excellent source of in-depth technical information that may not be published elsewhere