Yes, the one I created. I log hours through out the day and when I need to invoice I just select a date range and the invoice is created in PDF format automatically. I use it locally everyday and it happens to be the app I have applied to YC. It is called PointKit you can see a little preliminary screencast at http://pointkit.com/site When it is ready to accept Beta testers I will do an official announcement here.

I just use a Google Docs spreadsheet file to record when I arrive, when I leave, and when I go to lunch. (I have another pair of columns I can use for extended breaks, but I rarely use them and usually leave those columns hidden.)

I use OpenAir because my employer requires it. Not for billing or salary but rather to see how much time is spent on each project. Most importantly to distinguish development time from maintenance time.