Khan Academy - This free site provides numerous help vides with practice problems. You can assign specific topics, or the students can work in Mastery mode. Mastery mode tests all topics in your subject area. If your students add you as a coach, then you can monitor their progress.
IXL - This site is paid, but you can use the guest mode to practice problems on specific topics. Student earn a smart score, which goes up/down based upon their responses. If their solution is wrong, they are given the correct explanation. I use the Guest mode, which allows students to complete 10 or 20 problems for free. When they reach their daily limit, I tell them to clear their browsing history and this allows them to complete more exercises.
MathGames - This site contains 8th grade content that can be used for remediation. Topics include: Number Properties, Fractions, Decimals, Comparison, Algebra, Geometry, Ratios, Mixed Equations, Graphing, & Statistics.
ClassZone - This resource is aligned with the McDougal Littell and other textbooks.
YayMath.org - This site contains tutorial videos for Algebra 1 & 2, Geometry, Statistics, and Trigonometry.
Math.com - This resource includes applets, lessons, and practice problems to be solved online. This site contains K-12 content including Pre-Algebra, Algebra, Geometry, Calculus, Statistics, and more.
SoftSchools - Great site will all kinds of practice problems for every level of math.