I second iWeb, it is a great program. Now that I now that you just need a document template I would suggest FileMaker Pro or Microsoft Access to build a database for it. Using one of these database apps will allow you to keep detailed client information, tables of billable items and create nice looking invoices all in one package. They take a little time to set up, but once you have it, you never have to change it. I have attached an invoice that I made in FMP. I also have a worknotes template that I use, and somewhere I have a great followspot cue sheet template.
I use FileMaker Pro 8 Advanced, I believe they are up to 8.5 now. The standard version is $299 and Advanced is $499. You can get academic pricing at places like Academic Superstore. There is also a 30 day demo available from FileMaker.Do you have the FM application?