Go Math

National Council of Teachers of Mathematics: Grade-level appropriate resources and activities reviewed by NCTM members. Includes cross-curricular connections.
http://www.nctm.org/resources/content.aspx?id=8768

AAA Math: features a comprehensive set of interactive arithmetic lessons. Unlimited practice is available on each topic which allows thorough mastery of the concepts
http://www.aaastudy.com/

Internet4Classrooms: A collection of math activities starting ranging from Number & Operations to Data Analysis.
http://www.internet4classrooms.com/math.htm