Math Playground is a nice interactive site that was requested to me by one of my 6th Graders. I have hundreds of sites that help students practice and review math problems and functions, but this was not one of them.
Math Playground offers children in elementary and middle school many different math interactivities and games. Some of these games are created specifically for the site, others are embedded from other sites. The games are really good and there seems to be many different topics and standards covered. The site does have a few ads on it, but students are used to this.
Math Playground is a great site to help students review particular concepts you are covering in class. It is a great resource to extend your math curriculum outside of your classroom or to allow your students to play and surpass the time when they are finished with a class project. I have added this to my Math Resources to allow students to use the site when they are don with their work in my Computer Class.
Check out Math Playground at http://www.mathplayground.com/.