MyMood allows you to record moments of mood change throughout your day to day (via the ThumbUp and ThumbDown buttons).
You can then review graphs of said +/- data to help you make decisions, as well as help identify patterns in your mood (i.e. daily, weekly, etc.).
This app doesnt really do anything unless you use it regularly, as the goal is to capture enough data to allow you to make predictions and decisions based on the historical data!