Google Calendar based diary!
Ever look at the clock and wonder "what did I do all day"? I certainly have! But thanks to Reverse Calendar, I always know how I spent my time. Every hour, Reverse Calendar asks me how I spent the last hour, and I just type in whatever it was that I did. RevCal adds this information to my calendar. After a few days of this, I can go back and see exactly how I spent my time, which serves me two purposes:
1) I can learn about myself, and see how to improve my time management
2) Its nice to have a diary/journal of my life. Imagine being able to say exactly what you did last year, on Tuesday, April 4th at 2pm!
3) Its private to your own calendar, so only you have access to this information. (The code is open-source, so the community would definitely have found out if there were any suspicious activity going on in the code).