I had the school purchase interactive integers (0.99) for my classroom ipad set. I bought it for my high school students because many of my students need the scaffolding app can offer. It has two choices for practicing adding or subtracting signed numbers. One is color tiles and the other uses a number line.
The one that uses the color tiles has the student choose the tiles that represent the two numbers in the equation and then you solve it by matching up the positive and negative values. Before the student can match up the tiles, they have to make sure they have selected enough positive and negative tiles to represent the problem. After they verify the number of tiles, they can solve it and check the results to make sure they did it correctly. If at any point something is not write, the program lets them know that something is wrong and will not let them finish the problem till it is right.
The number line works in a similar way in that the students have to move the marker to the starting number, verify the position is correct and then they move the marker for the second part of the equation and they check the answer. Once they choose hard, the student is expected to complete the problems without the use of the number line or color tiles.
I think this app is geared more for upper elementary or lower middle school. I showed it to the sped dept and the teacher loves it for her students. Since I work with high school students I would love to see them have to type in the answer rather than relying on the computer to say it is correct and the answer is -2. I'd like to see the students input the answer and then have the app check it so students are better able to transfer the skills they learned from this app.