BambooInvoice is free Open Source invoicing software intended for small businesses and independent contractors. It has an Ajaxed interface (totally unobtrusive) with structured & meaningful XHTML.

BambooInvoice enables anyone to:

Add/edit/delete clients & related contacts,
Create invoices for clients, send them via e-mail, generate PDFs or print them, and
Define the payments made to the system to follow them

Also, a reports feature displays a financial snapshot of the given period.