GnuCash is personal and small-business financial-accounting software, freely licensed under the GNU GPL and available for GNU/Linux, BSD, Solaris, Mac OS X and Microsoft Windows.
Odoo (formerly OpenERP) is a suite of open-source business apps written in Python and distributed under the AGPL license, under the Open Core software business model. It is used by 2 million users worldwide to manage companies of all size.
ERPNext is a free and open-source integrated Enterprise Resource Planning (ERP) software developed by Frappé Technologies Pvt. Ltd. and is built on MariaDB database system using a Python based server-side framework.
Ledger is a powerful, double-entry accounting system that is accessed from the UNIX command-line. This may put off some users, since there is no flashy UI, but for those who want unparalleled reporting access to their data there are few alternatives.
Manager is free accounting software for Windows, Mac and Linux.
It features an intuitive and innovative user interface with modules such as cashbook, invoicing, receivables, payables, taxes and comprehensive financial reports.
The desktop edition is free while the cloud edition.
Tryton is a three-tiers high-level general purpose application platform under the license GPL-3 written in Python and uses PostgreSQL as database engine. It's the core base of a business solution providing modularity, scalability and security.
Insightly is a web-based project and Customer Relationship Management (CRM) application that integrates with Google Apps via a Gmail contextual gadget and Google Docs support.
The most popular version of Americas #1 small business financial software helps make your business more profitable.
webERP is a complete web based accounting/ERP system that requires only a web-browser and pdf reader to use. It has a wide range of features suitable for many businesses particularly distributed businesses in wholesale and distribution.