GnuCash - A personal and small-business financial-accounting software, licensed under GNU/GPL and available for Linux, Windows, Mac OS X, BSD, and Solaris.
Bench Accounting - Bookkeeping done for you
Synder - AR Automation
Xero - Xero is online accounting for your small business.
InvoiceApp - Beautiful online invoicing software for small businesses
FreshBooks - The ideal accounting software for small business owners.