According to the Common Core Standards, 8th grade math resources should focus on three critical areas: (1) formulating and reasoning about expressions and equations, including modeling an association in bivariate data with a linear equation, and solving linear equations and systems of linear equations; (2) grasping the concept of a function and using functions to describe quantitative relationships; (3) analyzing two- and three-dimensional space and figures using distance, angle, similarity, and congruence, and understanding and applying the Pythagorean Theorem.
If you know a particular Common Core Domain that you'd like to practice, you can drill down to it here.
(Click on a resource to see our full catalog)