Chima is a fun iOS app for wanting to reinforce students basic Math skills such as addition/subtraction. It's a fairly simple game where a user tries to answer a multiple choice Math question correct. It uses a basic fighting/adventure/RPG style game where a correct answer attacks/hits the enemy causing them to lose hit points (HP). The better a person does and they can level up. There's also a nice feature to choose the monster you want to go against.
I recommend checking out Chima by clicking here.
To try out the browser version of the game click here.