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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!

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    $\begingroup$ I edited your question a bit, my main goal was to remove the reference to WolframAlpha; and make the text easier to read. But feel free to revert if you like! (You can see the edit history and revert by clicking on "edited ... ago") $\endgroup$ – Ilya Nikokoshev Nov 4 '09 at 21:23
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    $\begingroup$ The answers below are great. Here's an idea I don't see that may be interesting to think about starting: An online example repository. This would be a place where one could upload and search various "first nontrivial example" notes. I think such a thing would really move mathematics along. $\endgroup$ – Jon Bannon Apr 20 '12 at 13:38

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Two sites created by my former wonderful A level Mathematics teacher: www.whitegroupmaths.com www.a-levelmaths.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

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A community database of rings examples, searchable by properties: http://ringtheory.herokuapp.com/

It is very similar to pi-base, which is a more famous database but for topological spaces examples.

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

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Sci-Hub 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.

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    $\begingroup$ You should probably mention it is the subject of a legal case, even if the jurisdictional issues complicate matters. $\endgroup$ – David Roberts Jun 8 '16 at 16:11
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I just found a very interesting site which has lots of free math videos even up to some more advanced topics:
http://www.hippocampus.org/

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http://www.math.fsu.edu/Virtual/

This site contains plenty of useful math resources.

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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

http://www.numbertheory.org/ntw/

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http://functions.wolfram.com/

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    $\begingroup$ We have that already - see above $\endgroup$ – vonjd Oct 27 '10 at 6:27
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For knot theorists, there are two really cool databases:

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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:

http://ccsl.mae.cornell.edu/eureqa

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

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