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.
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.
Games kids in grade pre-k through adult can play. However, most are geared toward elementary grades. It also has activity pages that can be printed for use in the classroom. d_3, gl_E, ev_4, y_f2
Various games in a virtual world. Although I did not stay here all that long, I did notice that the games seemed to offer an alternate way to memorize facts and did not really allow students to do any constructivist learning. Must be harder to build those games.
Fun games that let you practice reading call numbers and finding information. Was created to help student workers learn their jobs in an academic library.
This game would be more suited for Middle to High School students. Players are asked question about lasers when they reach a star in the game. Players are also asked to sort laser items from other items.