ProcrastriTracker is an open source time tracking tool that automatically tracks what applications and documents you use, and allows you to view statistics on your usage in great detail. It is written to be small and non-intrusive. It is useful for:
Increasing your productivity: seeing long term stats on what really is costing you the most time can help shift priorities to work more optimally.
Tracking and billing time: when working on multiple projects / for multiple clients, nearly automatically!
Fun! Seeing such detailed statistics is plain interesting, especially over longer time, even if you don’t intend to do anything with it.