This site has resources for various Math concepts

Patterns, Functions, and Algebra
This website will allow you to create free algebra worksheets.
http://www.math.com/students/worksheet/algebra_sp.htm

General Math Websites
Kuta Software will allow you to create free algebra worksheets.
http://www.kutasoftware.com/free.html

Homeschool Math has worksheets available for 6th grade math concepts as well as numerous middle school math concepts applicable to many grades
http://www.homeschoolmath.net/worksheets

Khan Academy has videos on concepts such as Arithmetic and Pre-Algebra, Algebra, Geometry, Probability, Statistics, and Brain Teasers. There is also practice available with the videos.
http://khanacademy.org

The following iXL websites provide practice questions specific to each grade
http://www.ixl.com/math/grade-6
http://www.ixl.com/math/grade-7
http://www.ixl.com/math/grade-8
http://www.ixl.com/math/algebra-1

AAA Math websites provide information and examples including practice problems to grade specific concepts
http://www.aaaknow.com/grade6.htm
http://www.aaaknow.com/grade7.htm
http://www.aaaknow.com/grade8.htm

Brightstorm provides videos on Algebra and Geometry concepts
http://www.brightstorm.com/math/algebra/
http://www.brightstorm.com/math/geometry

Brain Pop contains videos, practice, and activities based on many different math concepts
http://www.brainpop.com/math

Math Goodies features lessons and worksheets for middle school students
http://www.mathgoodies.com/

Math is fun provides games, puzzles, lessons, and online activities in such topics as algebra, geometry, measurement, and data
http://www.mathisfun.com

Other:
http://www.mathplayground.com
http://www.brainfun.com
http://www.crctlessons.com
http://math-play.com
http://www.sheppadsoftware.com
http://usmint.gov - go to the education tab, under kids games
http://www.quia.com
http://nlvm.usu.edu - National Library of Virtual Manipulatives