GregTangMath strives to provide unparalleled math lessons for students as well as professional development for in-service teachers. Games, puzzles, and other resources, like free downloads, worksheets/word problems, math centers.
This site, from the National Council of Teachers of Mathematics (NCTM), features complete lesson plans, mobile games for students, interactive activities, and brain teasers.
Khan Academy is on a mission to give a free, world-class education to anyone, anywhere. Their personalized learning resources are available for all ages. Video learning segments are followed by practice activities.
These math lessons emphasize the practicalities rather than the technicalities, demonstrating dependably helpful techniques, warning of likely "trick" test questions, and pointing out common student mistakes.