Invoice Ninja - Invoice Ninja is a free, open-source solution for invoicing and billing customers.
Invoice.to - A simple & beautiful invoice generator with Stripe & Paypal
Harvest - Simple time tracking, fast online invoicing, and powerful reporting software. Simplify employee timesheets and billing. Get started for free.
Free-Invoice.co.za - Free Invoice Software | Free online tool that helps small businesses to generate free invoices and manage their contact list. It can be used as an alternative to expensive invoicing software and online invoicing applications
GnuCash - A personal and small-business financial-accounting software, licensed under GNU/GPL and available for Linux, Windows, Mac OS X, BSD, and Solaris.
Invoice Template - Create beautiful looking invoices in seconds