I stumbled across AFactorTree by Walter Kissach. Although designed for the iPhone, it works quite well on the iPad. In fact, it is only found under iPhone apps but not iPad so it can be difficult to find.
This app is one I would use for scaffolding, differentiation and review because it is designed to give small chunks of information followed by examples and a special type of calculator.
The app uses a roller type list at the bottom so people can choose the topic they need to review. The photo shows the information on factoring trees. If you flick the list on the bottom up one, the topic will change to the building factor trees using the common approach.
The picture on the right shows what a student would see if they select building factor trees. It sets it up so the student knows that it requires 2 numbers.
If the student flicks to walk through, the answer is given so the student can check their answer before moving to the next problem.
In addition, there are two factor trees which will factor any number the student inputs. You can change the digits or change the whole number but the app automatically provides a factor tree. I can see this being used by the student to check their work.
This is not all the app covers. It also covers the least common multiples either using prime factorization or listing multiples, greatest common divisor, adding fractions, and signed numbers. This app is one that could easily be used throughout a pre-algebra class to reinforce, review, scaffold, differentiate instruction or to have students check their work.
Best of all, it is free. There are several other free apps in this series that I shall review over the next few weeks.