This site is devoted to mathematics and its applications. Created and run by Peter Saveliev.
Math online
From Mathematics Is A Science
Places to find math:
Type of site  Examples  Is it useful as a course?  Is it useful as a reference?  Is it fun? 

Video lectures  Khan Academy, MIT OpenCourseWare, YouTube  


Q&A/social  MathOverflow, StackExchange  


Online books  Applied Topology and Geometry, Hatcher's topology book  


Lecture notes  Online lecture notes  


Encyclopedias  Wikipedia, PlanetMath, Wolfram MathWorld  


Will there be time when everybody keeps his stuff at his own site and all of these "communities", aggregators, middlemen die out?