Kosmo - Kosmo helps you automate your freelancing business. Including sending invoices, tracking time, managing projects and more.
Invoice Ninja - Invoice Ninja is a free, open-source solution for invoicing and billing customers.
Harlow - Harlow helps freelancers get organized, save time, and look professional with automated invoicing, proposal templates, and much more—all from one centralized hub.
Harvest - Simple time tracking, fast online invoicing, and powerful reporting software. Simplify employee timesheets and billing. Get started for free.
Dubsado - Dubsado is flexible — it gives you 5 (now 6!) ways to add new leads. Best of all, 5 ways are automated.
GnuCash - A personal and small-business financial-accounting software, licensed under GNU/GPL and available for Linux, Windows, Mac OS X, BSD, and Solaris.