Free Textbooks

Matrix Analysis and Applied Linear Algebra
Abstract Algebra (an introduction to higher level mathematics with a concrete focus).

General Mathematics References

Wolfram Mathworld
MAA Online
American Mathematical Society
Wikipedia
Association for Women in Mathematics

Algebra

Algebra Resources
Arithmetic & Algebra Problems and Puzzles
Algebra Lessons

Calculus

Calculus Resources

Geometry

Geometry Resources
Geometry Resources2

Topology

Questions in Topology

Other Interesting Sites

Groups.dcs: a history of mathematics, list of mathematicians, and famous problems
Fractals
MATH=art?
Mathematics Education
Interactive Math
Upgrading your Mathematica program
The derivative game
The History of Calculus
Dimension Analysis (An example of the application of Math)
The natural logarithm functions
Topics in Calculus
Precalculus Problems (with solutions)
"Ask Dr. Math" : Math Help for all classes
Trigonometry Lesson
Geometric Analysis