ERP products usually cost a fortune, but for small firms and startups, such high prices are unaffordable. But there are several opensource ERP products what are as good as expensive commercial ERP products.
Below is the list of opensource ERP products which I could find, I have put them in random order.

1. Adempiere ERP

ADempiere Business Suite ERP/CRM/MFG/SCM/POS done the Bazaar way in an open and unabated fashion. Focus is on the Community that includes Subject Matter Specialists, Implementors and End-Users. We are a community fork of Compiere.

2. BlueErp

BlueERP is a double entry accounting application for small and medium business. Blue ERP is an open source project by its licence and by its philosophy. It is available as a stand alone application or as a set of modules for eGroupware

3. Dolibarr ERP

Dolibarr ERP/CRM is an open source/free software for small and medium companies, foundations or freelances. It includes different features for Enterprise Resource Planning (ERP) and Customer Relationship Management (CRM) but also other features for different activities.

4. ERP5

ERP5 is a free Enterprise Resource Planning system based on the Zope application server. It is mainly developed in the Python programming language and the source code is freely available under the GNU General Public License. ERP5 is developed by the French company Nexedi which also offers consulting, customizations and training for ERP5.

5. Compiere

Compiere is an open source ERP and CRM business solution for the Small and Medium-sized Enterprise (SME) in distribution, retail, service and manufacturing. Compiere is distributed by Compiere, Inc. and through the Compiere Partner Network, a collection of trained and authorized business partners.

6. GNU Enterprise

A set of tools, such as a data-aware user forms interface, a reporting system and an application server, which provide a development framework for enterprise information technology professionals to write or customise data-aware applications and deploy them effectively across large or small organisations. The GNUe platform boasts an open architecture and easy maintenance. It gives users a modular system and freedom from being stuck with a single-source vendor. GNUe supports multi-language interfa

7. GRR ERP

GRR ? Resource Management and Booking is a PHP/MySQL -based, web-accessed free enterprise resources planning software. GRR was written by a French team hence the acronym derived from its original name (Gestion et Réservation de Ressources), which can be translated as Resource management and reservation. It is available in 4 languages : French, English, German, Italian.

8. JFire ERP

JFire is an Enterprise Resource Planning and Customer Relationship Management system. The system has been written entirely in Java and is based on the technologies Java EE 1.4 (formerly J2EE), JDO 2, Eclipse RCP 3. Hence, both client and server can easily be extended and it requires only a relatively low effort to customize it for specific sectors or companies.

9. Kuali Foundation

The Kuali Foundation is a non-profit, 501(c)(3) corporation that coordinates the development of free/open source administrative software under the Educational Community License. The name “Kuali” came from the Malaysian word for wok. At a time when many colleges and universities are paying tens of millions of dollars for administrative systems, the notion of a wok — a humble, but essential dish — seemed appropriate.

10. LedgerSMB ERP

FLedgerSMB is a free software double entry accounting system. Accounting data is stored in an SQL Database Server and a standard web browser can be used as its user interface. The system uses the Perl language and a Perl database interface module for processing, and PostgreSQL for data storage.

11. Apache OFBiz ERP

Apache Open For Business (Apache OFBiz) is an Open Source Enterprise Resource Planning (ERP) system. It provides a suite of enterprise applications that integrate and automate many of the business processes of an enterprise. OFBiz is an Apache Software Foundation top level project.

12. Openbravo ERP

Openbravo is a web-based, open source ERP business solution for small and medium sized companies that is released under the Openbravo Public License, based on the Mozilla Public License. The model for the program was originally based on the Compiere ERP program that is also open source, released under the GNU General Public License version 2.

13. OpenERP

Open ERP (formerly named Tiny ERP) is an ERP/CRM system. The software is claimed to be a complete ERP and CRM system. It has separate client and server components. XML-RPC interfaces are available. Among the features are management accounting, financial accounting, inventory management, sales and purchase management, tasks automation, Human Resource Management, marketing campaigns, help desk, and point of sale.

14. Opentaps ERP

Opentaps is a web based ERP and CRM for small to medium sized businesses based on Apache Open for Business. Its functions include eCommerce, Point-of-Sales, inventory, warehouse, order, customer management and general ledger. It also offers built-in business intelligence tools and mobility integration, including with Microsoft Outlook, Google Calendar, and mobile phones. Opentaps has a web based UI (user interface). The server can be installed on a Windows 32-bit (2000/XP/2003) or any POSIX (Lin

15. OrangeHRM

OrangeHRM is a Human Resource Management (HRM) business solution for the Small and Medium-sized Enterprise (SME). It is developed on PHP, MySQL and Apache HTTP Server and can be downloaded to use on both the Linux operating system and Microsoft Windows. OrangeHRM is released under the GNU General Public License, and is thus free software.

16. Postbooks ERP

PostBooks is an open source accounting and enterprise resource planning business system geared toward small to medium sized businesses. It is released under a CPAL license and is thus free software. PostBooks is maintained as an open source project on SourceForge.net but is based on the commercially licensed xTuple ERP system created by xTuple, a private software company based in Norfolk, VA.

17. SQL-Ledger ERP

SQL-Ledger is a double entry accounting system. Accounting data is stored in an SQL Database Server and a standard web browser can be used as its user interface. The system uses the Perl language with a database interface module for processing and PostgreSQL for data storage.

18. Tryton ERP

Tryton is a three-tier high-level general purpose computer application platform on top of which is built a business solution (or ERP) through a set of so-called Tryton modules. The Tryton platform is organised around a three-tiers architecture: The Tryton client, the Tryton server and the DBMS (mainly PostgreSQL). The platform along with the official modules is licenced under the GPL-3.

19. WebERP

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. It is developed as an open-source application and is available as a free download to use. The feature set is continually expanding as new businesses and developers adopt it.