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 known as OpenERP, is an open-source business application suite developed in Python under the AGPL license. It follows the Open Core software business model and serves a global user base of 2 million.

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.

Dolibarr ERP & CRM is a modern and easy to use open-source web software package to manage your business (customers, invoices, orders, products, stocks, agenda, e-mailings, shipments...).

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.

Offers accounting software with payroll, bookkeeping, invoicing, app integration, and inventory tools. Four plans cater to businesses of all sizes. Automates reconciliation and shares with accountants seamlessly. Higher tiers add project tracking.

From invoicing to expense tracking to accounting, Akaunting has all the tools you need to manage your money online, for free.

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.

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 application that serves as a project management tool and a Customer Relationship Management (CRM) system. It integrates with Google Apps through a Gmail gadget and supports Google Docs.

Xero is a cloud-based accounting software for managing business finances. It allows users to send quotes and invoices, monitor cash flow, and pay staff from anywhere. It is suitable for small to large businesses, supporting complex accounting processes like bill payments...

FrontAccounting is a simple, but powerful, system for the entire ERP chain and covers: