Last week, I was looking for a virtual abacus for my older daughter when I found this app. I needed something easy to run, just to build numbers and explain her the difference between units, tens, hundreds and so on. Abacus is all this and much more because it structures step by step exercises with increasing difficulty. It is very intuitive, and the built-in tutorials help children to easy understand the use of the abacus through several levels. But the best of all: you can just play whith your children and see how fast they improve their calculus habilities, don’t using their fingers to count, but creating the mental structure to easily perfom additions and subtractions. The sound and music, and the included stamps for «certifying the learning process» makes it very attractive for children.
The app can be used in two ways: learning for young children, or exercising older ones through the more difficult level exercises. It is recommended for grades from 1 to 3, but I think it is very useful even for adults.
I strongly recommend this app specially for homeschoolers or very innovative teachers who want to include the iPad in their classroom. The free version is enough for learning up to two digits operations trough 12 levels of increasing difficulty, but it has an in-app purchase option to unlock all levels.