Wanted to shamelessly plug our Android SDK to anyone who may be interested. It's lightweight, easy implementation and best of all free to use. What it does is enable you to reward and keep track of your users with in-app badges. The SDK is also a great way to cross promote with other devs and their apps to increase your apps exposure even further beyond our own social graph - Everbadge - , which is currently at almost 90,000 members.
If you would like to take a quick peak or try out our sample solution you can see more at http://everbadge.com/developer
And this concludes my shameless plug