GnuCash - A personal and small-business financial-accounting software, licensed under GNU/GPL and available for Linux, Windows, Mac OS X, BSD, and Solaris.
Oracle NetSuite - NetSuite is the leading integrated cloud business software suite, including business accounting, ERP, CRM and ecommerce software.
Ledger-cli.org - Ledger is a powerful, double-entry accounting system that is accessed from the UNIX command-line.
Odoo - An all-integrated business app suite to unleash your growth potential.
Free Invoicing Software - Free Invoicing Software provides the facility of online billing.
Sage Intacct - Intacct is a cloud-based accounting and financial management system that is designed for the businesses of all levels.