AnkiDroid is the best spaced repetition flashcard software for mobile. Period.
It’s more customizable than others out there, free as in freedom and as in cash (at least the Android version), and, most importantly, it works wonders for learning and memorization.
One thing that could be improved is allowing plug-ins to extend functionality of the app, like the desktop version does. Also, some actions can only be done via the desktop program, which makes it a second-class citizen for me. The fact that it can sync seamlessly across devices makes it a smaller problem, however.