RbInvoice lets you generate PDF invoices from a Google Spreadsheet. It's pretty obscure; you probably haven't heard of it.
RbInvoice is not production-ready code! I keep it on Github more for my own convenience than anything else. I do use it myself to bill clients, but lots of things are hard-coded, like my company address. If you use it, you do so at your own risk! I don't guarantee anything, and I don't promise any support. If you tell your clients to write checks to Paul Jungwirth and send them to my address, that's too bad for you. :-)
Perhaps someday I'll get this code into a shareable state; it's inching there a little bit each month. But right now you should find a real invoicing solution somewhere else. All documentation here is purely in expectation of an eventual release. Things may be broken and may change, so please don't take it as a promise of anything.
RbInvoice reads your hours from a Google Spreadsheet, which should be formatted like this:
| My Time Tracking | ||||||
| Weekday | Day | Task | Notes | Start | Stop | Total |
| T | 3/20/2012 | BigCorp | API | 8:00 | 12:15 | 4:15 |
| T | 3/20/2012 | SmallCorp | Shopping Cart | 13:00 | 17:15 | 4:00 |
Columns B, E, F, and G should have a Date format. I calculate G automatically by saying =max(0, F3 - E3), but if you do that, make sure you enter times in 24-hour format, because if you work through lunch (e.g. 11:00 to 1:30) your total column will be 0:00.