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! 


Two sites created by my former wonderful A level Mathematics teacher: www.whitegroupmaths.com www.alevelmaths.com He has generously written tons of topic summaries, worked revision problem sets and other learning material made mostly free to us students. Felt he deserves a mention for all his efforts :) Thanx n hope u will benefit from them! Estella 


LMFDB has been officially launched yesterday (10th May of 2016). It is an integrated knowledge database of Lfunctions and modular forms with a nice web interface that helps visualize the connections between these mathematical objects. 


For people who are interested in prime factorization : http://www.mersenneforum.org 


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





For knot theorists, there are two really cool databases: 


SciHub is pretty helpful in accessing articles, even for those researchers who already have access to several journals. The interface is great, the site is pretty fast, and the database is huge. See this article and other linked articles there for a nice overview of who all are downloading pirated papers. Edit: as pointed out in the comments, it should be noted that there is an ongoing lawsuit against the website. 


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/ 


A community database of rings examples, searchable by properties: http://ringtheory.herokuapp.com/ It is very similar to pibase, which is a more famous database but for topological spaces examples. 


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