http://do2learn.com/activities/mathhelpers/index.htm
XtraMath (create a class, add students and get pin numbers to use as their passwords - all free)
Math - All ages
Math Logic - fun for grades 2-6
http://www.coolmath-games.com/0-bloxorz/index.html
Math - Grade 6 resources
http://www.masters.ab.ca/bdyck/math/biography/
http://www2.ups.edu/community/tofu/lev2/journaling/writemath.htm
http://www.mathwire.com/writing/writing1.html
HoodaMath
Alberta Math Interactives - Excellent
http://www.learnalberta.ca/content/mejhm/index.html?l=0
Khan Academy
It has videos and practice questions for all sorts of subjects - mostly math. A lot of the content is HS but there are some videos that would be appropriate for ELEM and MS.
National Library of Virtual Manipulatives
http://nlvm.usu.edu/en/nav/grade_g_2.html
http://www.tusd1.org/resources/math/students_math.asp
Numbers and Operations
ABC of Math - All levels
http://abcofmath.com/math-sites/lower-elementary/l2m.htm
Fun 4 The Brain
http://www.fun4thebrain.com/index.html
Math Nook
A nice tool to use whole group for Science or Math.
http://studyjams.scholastic.com/studyjams/index.htm
Welcome to Math Playground, an action-packed site for elementary and middle school students. Practice your math skills, play a logic game and have some fun!
http://www.mathplayground.com/index.html
http://math.pppst.com/division.html
http://www.tenmarks.com/tmother/teacher-index
Other
Other sites in one to check out