The other day, I found a nice app on iTunes called Algebra A +. The basic app is free but if you want more than the complementary problems, you'll have to upgrade by paying $3.99.
This app offers exercises covering addition and subtraction of integers, substituting values into an expression, combining like terms, distributive property, binomial multiplication, factoring a monomial, difference of two squares, factoring by grouping and factoring quadratics.
For each problem, there is a pencil up in the right hand corner that when selected, brings up a white board that covers the right side of the page so you can still see the problem.
The white space allows for the student to work out the problems. Once the answer if gotten, the student can then type in the solution. If the solution is correct, a green check mark appears in the left corner. If its wrong, the correct answer appears in a pop up dialog box.
The other thing I don't like is that it does not show a student how to do the problem if they should get it wrong.
I do like the number of problems a student gets with the free app. I also like that there are some very good topics to choose from. This app provides good practice problems for students who want the extra practice without hauling a book around with them.