I'd like to be able to track time against my client list

I'd like to be able to track time against my clients for billing purposes. It would be great if there could be a tray or sidebar application that I can click on to say what client I'm working for at any time (from a list that I can build myself) and then for any activity tracked during that time to be auto-tagged with the clients name. That way I can see how much time I'm spending working for each client and hopefully if hour-by-hour reporting is implemented I can use this information to prepare client billing.