The ERP market is dominated by giants like SAP and Oracle, but companies interested in open source ERP do have options.
ADempiere ERP Business Suite. (Java) ADempiere is a comprehensive multi-platform open source ERP platform that was forked from another ERP project called Compiere back in 2006. It offers a vast array of features including sales, purchasing, accounting, manufacturing management, material management and finance, as well as HR, CRM and payroll. The software is available to download from Sourceforge under the GPL.
Apache OFBiz. Multi-platform (Java) OFBiz (which stands for Open for Business) is the Apache Foundation's enterprise business package. Released under the Apache License 2.0, it is free to download from Apache. The suite includes many ERP functions including e-commerce, catalog management, promotion and pricing management, order management, warehouse management, accounting, manufacturing management, PoS and other features.
xTuple (Qt and Postgres) claims to be the world's number one open source ERP suite and has been around in one form or another since 2000. It offers a free open source edition called PostBooks which is available from Sourceforge, and is also available as a licensed product with maintenance, support and the option to use it as a service offered from the cloud.
As well as these two editions, which are aimed at small and medium sized companies, xTuple offers three commercial editions for larger organizations - one aimed at distributors, one targeted at manufacturers and a more general enterprise edition. These are built on the same open source foundation and share the same code base.
The open source edition includes functionality for accounting, sales, CRM, purchasing, product definition, inventory and distribution, light manufacturing and the OpenRPT open source report writer. The commercial license for PostBooks costs $1,200 per user for a perpetual license, or $400 per user per year. If used as a service this costs an extra $100 per month, plus $30 per user per month.
The community edition of Odoo is available to download for free, and this includes all modules - ERP ones as well as CRM, marketing and others. The hosted version is available free for two users, and thereafter costs Euros 12 per user ($15 U.S) per month for each app, including email support. A more comprehensive package that includes customization assistance and training materials is also available for Euros 111 per user ($138 U.S.) per month for each app.
OpenBravo (Java) is an open source ERP system which is available to download on Sourceforge. Aimed at small and medium sized businesses, this modular system includes finance and accounting, purchasing management, sales management, inventory management, project management, PoS, business intelligence and other modules.
Openbravo also offers two commercial editions -- a professional edition for smaller companies with up to five concurrent users and an enterprise edition for larger companies with substantial numbers of users.
Opentaps is an integrated open source ERP and CRM suite sponsored by Open Source Strategies, Inc. and used by organizations including Toyota and Honeywell. Opentaps supports e-commerce, CRM, warehouse and inventory management, supply chain management, financial management, business intelligence and mobility integration out of the box. The name "opentaps" was originally an acronym for "Open Source Enterprise Applications Suite."
opentaps is available as free software under the GNU Affero GPL v3, and also with a commercial license that allows you to modify the source code without the obligation to make your customizations public.
The opentaps Professional Edition is offered under a commercial license with a support package provided by Open Strategies that includes updates for opentaps releases, access to opentaps support issue tracker, guaranteed response time of four hours during support hours and remote assistance via VNC and SSH for $600 per user per year.
Dolibarr is an open source business suite designed for small companies and individual traders that includes ERP and CRM functionality. It can be installed on a local machine or accessed from a server, and is also offered as a software-as-a-service (SaaS) solution with a free trial period by Dolibarr-preferred partners including DoliCloud and DoliCloud and ATM Consulting.
The open source ERP software is highly modular, and main modules include sales and purchase management, stock management, bank account management, orders management and shipping management. Additional paid-for modules are also available from the official DoliStore.
ERP5 (Python, Zope) offers open source ERP that includes accounting, customer relationship management, trade, warehouse management, shipping, invoicing, human resource management, product design, production and project management.
ERPNext (MySQL, Python) is an open source ERP solution similar to Openbravo and Odoo, aimed at small and medium sized businesses.
LedgerSMB (PostgreSQL, Perl) is a free double entry accounting and open source ERP software system aimed at small and medium sized businesses.