In MatematicasVisuales we try to see mathematical concepts. If you want to learn more about the ideas involved you can find useful information in
It is a very big site about mathematics with visualization applications made using Mathematica.
The web site 'Cátedra UCM Miguel de Guzmán' has a lot of information about Miguel de Guzmán.
Web site with a lot of pages and interactive applications about Mathematics developed by Alexander Bogomolny.
Excellent mathlets using GeoGebra.
Videos and mathematical animations:
A wonderful book.
Farris introduces a new way to use color in visualizing complex-valued functions in the plane.
He has a very famous video about Moebius functions: Möbius Transformations Revealed.
Excellent course with lots of interactive applets (spanish).
Convergence is the Mathematical Association of America free online journal about the history of mathematics and its use in teaching.
A very interesting TEX and LATEX resource.