A wonderful tool for reviewing math both in school and at home though it does not introduce new concepts.
Community Review for Sumdog
Sumdog is not teaching new content, but it is an excellent review tool. While you may have a student that begins to understand new concepts while playing Sumdog, it is not a teaching tool for those that want to introduce new content. Because Sumdog will continue to introduce new concepts until a student hits a wall, students will occasionally be forced into answering questions they have never seen before (ex. Multiplication or division when they haven't learned it in the classroom yet.) Sumdog seems to be easy to contact through email, but I don't have a lot of experience with this.
TLDR: A lot of fun for students and a very welcome math review tool for students to use at home and at school.
How I Use It
Initially confronted with paying for a math-based program, I turned to Sumdog as a free alternative. It allows for unlimited student accounts and access to most games for free. The recent changes that allow the website and teacher to customize student levels are very welcome. You no longer have to set everyone on the same level, Sumdog helps find their level as they work. There are a lot of incentives for students to continue playing the math games; from houses to pets to avatar clothing and game re-skins. Students also greatly enjoy playing against each other. The pay wall only prevents teachers from running reports and locking students out of one game.