Free Invoice Generator - A free invoice generator for startups
Invoice Ninja - Invoice Ninja is a free, open-source solution for invoicing and billing customers.
Invoice Maker - Generate your invoices online for free
Harvest - Simple time tracking, fast online invoicing, and powerful reporting software. Simplify employee timesheets and billing. Get started for free.
Billdogg - Beautifully simple invoicing for freelancers
GnuCash - A personal and small-business financial-accounting software, licensed under GNU/GPL and available for Linux, Windows, Mac OS X, BSD, and Solaris.