Eric Weisstein's Treasure Troves of Science - Contains the extensive archive of the biographies of the famous scientist. The best part of the biography is that it contains links to the explanations of the scientific concepts developed.

How Stuff Works - has numerous articles which explores the workings of everyday device from phones to refrigerators, and also explains the principles behind concepts such as helium and pi in a very clear and easy to understand way.

Academic Press Dictionary of Science and Technology - Academic Press in association with Harcourt brings scientific dictionary with more that one hundred thousand  scientific terms.

xrefer - is a reference search engine contains encyclopedias, dictionaries, thesauri, books of quotations and reference works covering a wide range of subjects including science and technology.