FreshBooks - The ideal accounting software for small business owners.
Invoice Ninja - Invoice Ninja is a free, open-source solution for invoicing and billing customers.
Zoho Books - Smart Accounting for Growing Business
Harvest - Simple time tracking, fast online invoicing, and powerful reporting software. Simplify employee timesheets and billing. Get started for free.
Paymo - Manage projects, track time, create invoices, and get paid online.
GnuCash - A personal and small-business financial-accounting software, licensed under GNU/GPL and available for Linux, Windows, Mac OS X, BSD, and Solaris.