I am tutoring close to home and one student LOVES the Pizza 1 App. It is a great fraction app for beginning fractions. Number Math App is also a great tool for different skills such as skip counting, rounding and missing number. MeeGenius has free books that read outloud. The student I tutor really enjoyed listening to the story. Spelling Bug works on CVC words. The letters are mixed up and they have to drag the letter and put in correct order.
For older students, I found some multiplication and division apps. Play 19x19 works on multiplication facts. MathEdge is an interactive long division app that guides the student through the problem.
Just search the skill in the App Store and a lot of choices appear. Good luck!