FlashToPass
I have been using this Math app for awhile and the students are improving their basic fact knowledge.  The digital flashcards review all four operations with a 2 minute timer.  The levels can be set from easy to hard.  For students who are intimidated by the timer, this is great because it continues into negative time.  The students continue until they get frustrated, or I tell them to stop.  You can manually track their progress, or have students log/graph their score.