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! 


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 


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


Jahrbuch Database A sort of Mathscinet and Zentralblatt for the period 18681942. Most of the reviews are in German. It is interesting to read the reviews written by mathematicians like Frobenius, Hilbert, Minkowski, Hasse, E. Noether, Artin, MittagLeffler, Landau, Van der Waerden, ... 


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


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

