gTime creates simple invoices by reading from your Google Calendar. There are screenshots of creating an invoice on the project hosted at Google Code.
The project was a birthday gift for my wife last year, but it was also a hobby project to explore Google App Engine and GData APIs. The primary motivations for the app were to:
- eliminate redundant data entry by generating invoices directly from the appointments you already keep current on your Calendar.
- work with any Calendar client by reading billing rates from plain text in the calendar entries themselves.
- protect client confidentiality by printing Web-based invoices that do not pass identifying name and addresses over the Internet.
That last goal is a bit specific to healthcare providers, but expand the app for your own needs (and maybe even punch out a few features on the wish list)