MCAT Prep has thousands of questions already, and adds a question a day. However, I find their questions to rather simple. MCAT Flash has a bunch of flashcards made by others as well, which seems pretty good!
I would definitely like to know about some useful apps that you pay for though! The credibility of all the free ones can sometimes be questioned, but if I payed money for an app I get some reassurance on the quality of the content.