The other day, I was looking for a a good task timer I could use. I passed many apps including Harvest and SideJobTrack which were too focused on the job and invoicing part of it. Ten sign-ups later, I concluded that my favorite two are SlimTimer and Toggl. Both are great timers, but they are distinctly different, and will appeal to different people. SlimTimer has more features, such as a sidebar bookmarklet, but Toggl seems to have a simpler and easier to understand interface, specifically the task pane. Ultimately, the decision is up to you. Try both, and tell me what you think.
If you do not like that they are web-based, you can run them on your desktop with Bubbles. Bubbles basically takes web apps, gives them their own window, and lets them be minimized to system tray.