Apple - Is there an app that allows me to monitor time spent using an application?
Timing helps in tracking time spent in applications (and their windows) as well. It's similar to Time Sink but seems to provide more grouping and reporting options.
The application Time Sink ($5, Many Tricks) tracks the amount of time you spend in each application (and each window within an application).
TimeEdition is an application that I just found. I haven't used it yet, but it's free. However, it doesn't have as many features as Time Sink.