Microsoft, SAP, Oracle are leaders when it comes to ERP software systems, but they come with huge upfront costs and lack of customizability because of their closed source nature. There are some good opensource aleternatives to these ERP systems.
There are a number of flexible, feature-rich, and cost-effective open source ERP systems available in the market. Here is a list :
ERPNext is a free and open-source integrated Enterprise Resource Planning software developed by Frappé Technologies Pvt. Ltd. and is built on MariaDB database system using a Python based server-side framework
Odoo is an all-in-one management software that offers a range of business applications that form a complete suite of enterprise management applications targeting companies of all sizes
webERP is a complete web based accounting and business management 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, distribution and manufacturing
opentaps Open Source ERP + CRM is a fully integrated application suite that brings together top-tier open source projects to help you manage your business more effectively
Apache OFBiz is an open source enterprise resource planning system. It provides a suite of enterprise applications that integrate and automate many of the business processes of an enterprise.
ADempiere Business Suite is an industrial strength open-source software solution that combines ERP, CRM and SCM support for business process. ADempiere provides a framework for extending and customizing to meet business needs