I have been using the Pull-ups App by Rittr Labs on my Android. The app gives you a daily pull-up workout routine that will gradually build you up to 25 reps at once. I can definitively see an improvement since beginning the program.
Check them out at: https://market.android.com/details?id=com.rittr.pullups&hl=en
Here are some more apps by Rittr Labs for other exercises: https://market.android.com/developer?pub=Rittr+Labs