The wonderful world of Freshbooks! It’s free if you’re only using it for 3 clients, but if you just want to use it for time tracking purposes, that’s even easier.
If you are running on a Mac, you can download their little widget thing for your dashboard. My time tracking is super easy now, two steps:
1. Hit F12 (brings up dashboard)
2. Hit "start timer" on the widget
You can write notes about what you’re doing during that time, select client, project, etc. Once you’re done, hit "submit hours" and they get uploaded to your account online. They even have a free app for the iPhone for time tracking on the road (I use it when I do work on a plane or ferry, or if I go to a meeting, etc etc).