The Best Math Websites

Math websites that come up frequently in the life of a research mathematician or math grad student.  Note that Wikipedia is also an excellent resource for a wide range of surprisingly specialized math topics and proofs. 
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MathOverflow Ask the math community your research-level math questions or start a general interest math discussion. Many big math names are heavy users. MathOverflow.net home page MathOverflow FAQ  
arXiv Self-serve moderated archive for posting high-quality research preprints and expository articles in math and related fields. Used more widely in some areas of mathematics than others. arXiv Home Page Getting started with the arXiv Cornell University 
Math Stack Exchange Like MathOverflow, but for the student demographic. http://math.stackexchange.com/  Stack Exchange 
Physics Forums Popular forum for math and physics questions. Non-research math level, mostly high school and college course material. http://www.physicsforums.com/   
Open Problem Garden Get inspired! Browse unsolved research-level math problems. Homepage List by keyword Simon Fraser University 
Art of Problem Solving Heavily trafficked forum for solving math problems. Particularly geared toward high school math competition problems. Forums homepage   
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