KidsCalculate helps children to develop a mental image that helps them to understand preferred methods for solving mental calculations. It treats comparison, counting, addition, subtraction, multiplication tables and learning numbers in 9 foreign languages.
I think the KidsCalculate system may be the best in the world. A detailed comparison with other calculation visualisation systems (like the Abacus, Bead String, Number Line and Montessori Stacking System) can be found at:
http://www.kidscalculate.com
Although the focus is primarily educational, some fun is added as encouragement: Mini-games, cut-scene movies, prizes that can be won and crate animations.