At Disney Interactive, I had the opportunity to build two unique achievement systems, both were great learning experiences. I've seen achievement systems at work in plenty of games, but it's interesting to draw parallels and observe the "gamification" of features at work in health and lifestyle apps.
^ The Nike+ Running app has a nice system that encourages reaching fitness milestones
^ The Blackwood & Bell Mysteries achievement system was built to increase retention and monetization
^ In Disney's Words of Wonder, achievements were built to increase retention, and provide additional gameplay for elder users.