Timeboxing is a great way of breaking your work up into manageable chunks. The key idea is you work non-stop for some time (be it 5 minutes, or 2 hours), followed by a short break. Rinse and repeat.
This app will keep track of your completed timeboxes, and display this data. These timeboxes are only accounted for if the timer runs to completion. The app works fine in Chrome and Firefox. Unfortunately, not in Safari.
For every 30 minutes you work, the calendar will add a red dot.
The Bubble Chart
Every time you complete a timebox, the relative frequences are computed and displayed here. You'll get a better idea of when you're most productive.