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National Science Foundation: An independent federal agency created by Congress in 1950 "to promote the progress of science; to advance the national health, prosperity, and welfare; to secure the national defence."
Science.gov. Gateway to a large selection of websites, narrowed by topic.
Science Research: A free, publicly available deep web search engine that uses advanced "federated search technology" to return high quality results by submitting your search query - in real-time - to other well respected search engines then collating, ranking and dropping duplicates of the results.
Scitopia. Digital library of 21 leading science and technology societies.
Wolfram Mathworld: The web's most extensive mathematical resource, provided as a free service to the world's mathematics and internet communities.