Last year I had the opportunity to hear Greg Tang speak about math!
Greg Tang is the author of such books as:
He has an amazing FREE website: www.gregtangmath.com.
There is also an app for each operation if you choose to buy the apps, but the online version is free. Your students can create a link to the game that looks like an app by adding it to their Home Screen. Just click on the blue box with the arrow at the top of the screen on the specific website so that it can be added to the Home Screen.
My students love these games, and the positive reinforcement given by the timing and points really motivates them to keep practicing!