Here is another site dedicated to understanding and solving math problems. What's interesting about this site is that English and Science are incorporated as well. It's a known fact that these 3 subjects are dependent upon one another and understanding these subjects (as a whole) is essential. This site is geared for the primary grades.
what's really 'cool' about this site is the use of word problems which are often difficult for someto grasp the understanding of completing such a problem.
Although not a free site, Zoodles has a great deal of interactive games for the various primary school age subjects. My 10-year-old enjoyed the free trial so we're considering purchasing a membership for 1 year. Subjects include reading, math, science, and writing.