GnuCash - A personal and small-business financial-accounting software, licensed under GNU/GPL and available for Linux, Windows, Mac OS X, BSD, and Solaris.
vatlayer API - Free VAT Validation & EU VAT Rates API
Xero - Xero is online accounting for your small business.
MFLicenseCenter.com - Connect to fuel tax licenses for tax determination, return preparation, and reporting compliance.
FreshBooks - The ideal accounting software for small business owners.
TaxJar - TaxJar simplifies sales tax filing for eCommerce sellers.