Mappiness
Mappiness is a free iPhone app that's part of a research project at the London School of Economics. The app prompts you a few times a day to ask how you are feeling, who you are with, where you are, and what you are doing. The data is anonymously collected by the LSE, who are analyzing it in particular for the effect of local environment (including noise) on people's mood. Users can view their own happiness history directly in the app.