The Abacus (http://www.ee.ryerson.ca:8080/~elf/abacus)
An excellent site devoted to the abacus, which includes history, directions, and teacher resources.

Ask Dr. Math (http://forum.swarthmore.edu/dr.math/)
A program of Swarthmore College, where K-12 students can send math questions to be answered. Includes an archives and a math library.

Allmath (http://www.allmath.com)
Links to great math materials on the web, including fun and games, kid references, math articles, help, and teacher resources.

Curious Math (http://www.curiousmath.com)
Tips, tricks, tools, and entertainment with math.

IXL Math Practice

Khan Academy's Common Core Resources

Khan Academy
Videos on every math and science topic.

Math Forum : Class2Class (http://mathforum.com/class2class/)
A clearinghouse of collaborative projects, data-collecting experiments, peer tutoring, and keypal exchanges designed to enhance the mathematics in your math, science and interdisciplinary curricula.

MegaMath (http://www.c3.lanl.gov/mega-math/index.html)
Developed by Los Alamos National Laboratory, this is a collaboration with teachers, students, and parents to bring important mathematical ideas to students. Creative and fun!

Proof Wiki

SOS Math Algebra

Virtual Nerd
Lessons on many topics from algebra to geometry.

Webmath (http://www.webmath.com/)
Find the type of math you need help with, and then follow the instructions -- help with everything from fractions to alegebra.

Wolfram Alpha