Eng-Tips. Forums for engineering professionals of every kind, including aeronautic, automotive, chemical, civil/environmental, computer, electrical/electronic, geotechnical, industrial/manufacturing, materials and mechanical. Also has information on standards and software.

MadSci Network. Scientists providing answers to your questions. Also has some interesting oddities, faq's and archives.

Straight Dope. Answers to common vexing questions in science and life in general. Track down urban legends and find out if what they say about Catherine (The Great) is really true. [No - it's not.]