Two of our games use Scoreoid :-
charmingly Tap Shoot Zombie
Try them. They are free.
Also if you are considering adding high scores, achievements etc in your game then you should try out Scoreoid. Its cross platform, uses web services to handle scores and also as it has no sdk so it does not increase the size of the application as well. Finally it is really easy to implement also. So try it out.
Our latest game: