Simple Invoices - Never Wait For Clients To Pay You Again!
Invoice Ninja - Invoice Ninja is a free, open-source solution for invoicing and billing customers.
Express Invoice - Express Invoice is the easiest system for creating and tracking quotes, orders and invoices for your business. Use web access to view your data from anywhere.
Harvest - Simple time tracking, fast online invoicing, and powerful reporting software. Simplify employee timesheets and billing. Get started for free.
FreshBooks - The ideal accounting software for small business owners.
GnuCash - A personal and small-business financial-accounting software, licensed under GNU/GPL and available for Linux, Windows, Mac OS X, BSD, and Solaris.