I use two flashcard apps called “Chinese Flashcards” and “Chinese Flashcards II” which are pretty great. They both cost $5 I believe, but they’re really good. The first one is HSK characters and the second is HSK words and short phrases. It’s an intelligently-spaced algorithm, a la Anki, so it tests you most on the characters that you’re weakest at.
I’ve tried out a lot of other flashcard apps, the problem is usually that the flashcard interface sucks, or the wordlist is too short or cannot be changed.
The Nciku app is good because it provides a lot of example sentences, which nothing else I’ve seen does. For $5 it comes up a little short considering it doesn’t even have a wordlist or flashcard feature, but I’m hopeful that they bring it up to the Nciku.com level of quality.