GnuCash - A personal and small-business financial-accounting software, licensed under GNU/GPL and available for Linux, Windows, Mac OS X, BSD, and Solaris.
Synder - AR Automation
vatlayer API - Free VAT Validation & EU VAT Rates API
Xero - Xero is online accounting for your small business.
Transiteo - Calculate customs duties and taxes on your online store.
FreshBooks - The ideal accounting software for small business owners.