search for "gamification". Gamification is the term generally used for systems that track what users do and then reward them for doing them. There are companies that specialize in add-ins for this type of feature.
You might also look at what software you are using to drive your website. There might be add-ins for the website software you are using that adds gamification features. For example, this forum software has a number of features that reward people with badges or other abilities based on certain things happening -- such as number of posts, ratings, time on site, etc.