Algebra Touch is a Great App.
I am a teacher and have used this app with several students to try it out. I love this app. It's very affordable and does a great job helping kids solve problems. Students should practice telling you what they are doing to solve the problem to check their learning/strategies. They learn to simplify (condense/collect like terms, eliminate), factorize, substitute, and distribute. It also helped me brush up on skills I hadn't used in years.