Media Atelier might have an answer in their new Leopard-only invoicing package for Mac, GrandTotal. GrandTotal provides a number of design tools for creating a good-looking invoice to represent your organization, and free templates are available from Meda Atelier's website. You can then set up your customers, create a catalog of goods or services, create estimates or full invoices, and send them out either immediately or on a timed basis. Invoices are marked off when paid, and the Dock icon always shows how many invoices are due or overdue.
GrandTotal works hand-in-hand with Media Atelier's TimeLog 4 application, so for those of you who work on a billable hour basis can enter your time into the TimeLog app, then import into GrandTotal. A free trial is available for download (click downloads file), and the application can be purchased from within the demo for €49.
What do you use to create invoices? Leave a note in the comments.