Rescuetime runs in the background with minimal unnecessary interactions. It keeps track of what windows I'm working in, including what sites I visit. It gives me feedback whenever I want it to keep track of my productivity or to identify where I'm wasting time. It's customizable in terms of categories and how things are assigned to categories--e.g., for me, a certain program that would normally be considered a distraction is classified as physical exercise because I only use it while riding my bike.

It's great to see the actual numbers. There's a clear increase in productivity.

I would not have finished my dissertation without RescueTime. Everything about it is essential. It automatically tracks my on-computer time, gives me the option of tracking off-computer time, and sends me stats each week. With the paid version you also have the ability to block applications/ websites that drain your productivity.