Khan Academy (Log-in or scroll down and browse by grade/topic)
Virtual Manipulatives
Math Wire (Math activities in every strand!)
Math is Fun
Area of Plane Shapes (Math is Fun)

More about area of Triangles
Free Math Help

Games
Mathplayground
Softschools Math
Cyberchase (Math challenges from the PBS Cybersquad!)

Multiplication Focus
www.multiplication.com
Math Playground (mult. only)
Math Mania Timed Test
Math Quick Quiz
Xtramath.org