What are really helpful math resources out there on the web?
Please don't only post a link but a short description of what it does and why it is helpful.
Please only one resource per answer and let the votes decide which are the best!
What are really helpful math resources out there on the web? Please don't only post a link but a short description of what it does and why it is helpful. Please only one resource per answer and let the votes decide which are the best! 


www.proofwiki.org It is a wikipedia, for proofs. 


OntoMathPro, a crowdsourced ontology of professional math knowledge. 


I just found a very interesting site which has lots of free math videos even up to some more advanced topics: 


http://www.math.fsu.edu/Virtual/ This site contains plenty of useful math resources. 


If you want to find a relationship between data in the form of closed form formulas this tool is  to the best of my knowledge  the best one: 


An excellent catalogue of mathematical information available on the web is Keith Mathhews' http://www.numbertheory.org/ntw/gateways.html and if you are interested in Number Theory, see 


NPTEL provides Elearning through online Web and Video courses in Mathematics organized by Indian Institute of Technology. http://nptel.iitm.ac.in/ 


For knot theorists, there are two really cool databases: 


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