MITx - https://www.edx.org/
Geometry
IXL Geometry - http://www.ixl.com/math/geometry
Calculus
Paul's Online Math Notes - http://tutorial.math.lamar.edu/
Problem Solving
Art of Problem Solving - http://www.artofproblemsolving.com/Wiki/index.php/Academic_competitions
Places to Visit
The Museum of Mathematics - http://momath.org/