Power Time is a time management system written in Python which uses the Django web framework for access and information display. It allows you to:

• Charge different clients different hourly rates.
• Charge for different rates based on employee working on a project.
• Charge hourly or set prices for different projects.
• Automatically notify your clients when their bills come due.
• Track payments received, paychecks sent, and bills paid.
• Keep track of communications with clients.

And much more. If you know Python and Django, it's also very easy to customize and extend.