Computational Knowledge Engine
http://www.wolframalpha.com/

Mr. P's Math Page
http://www.pleacher.com/mp/mpframe.html

Math Chimp
http://www.mathchimp.com/

Pre-Algebra:


Math.Com Pre-Algebra
http://www.math.com/homeworkhelp/PreAlgebra.html

Pre-Algebra--a complete course of study:
http://www.free-ed.net/free-ed/math/prealg01/default.asp

Cool Math: Pre-Algebra
http://www.coolmath.com/prealgebra/index.html



Algebra:

Math.Com Algebra
http://math.com/homeworkhelp/Algebra.html

National Library of Virtual Math Manipulatives: Algebra:
http://nlvm.usu.edu/en/nav/topic_t_2.html

Interative Game for Algebra:
http://regentsprep.org/REgents/math/ALGEBRA/games/Aquiapage.htm



Geometry:

National Library of Virtual Math Manipulatives: Geometry:
http://nlvm.usu.edu/en/nav/topic_t_3.html

Math.Com Geometry:
http://math.com/homeworkhelp/Geometry.html


General or Mixed:

7-12 Grades Math Interactives
http://www.uen.org/7-12interactives/math.shtml

Illuminations: Math Activities
http://illuminations.nctm.org/ActivitySearch.aspx