Is open source software legal?

Yes, and almost everyone who uses e-mail or surfs the Web is an open source software user. Most e-mail passes through an open source server during its travels across the Internet. Most Web servers rely on open source software. Google is based on open source. As OSI points out, the running gears of the Internet, including the mail transports, Web and FTP servers, are virtually all open source-based.

Open source software is not warez, which is software distributed without permission of the copyright holders or a proper license.

Open Source ERP - Few offerings by community

Open for Business

The Apache Open For Business Project is an open source enterprise automation software project licensed under the Apache License Version 2.0. Several ERP systems are based on OfBiz, including Neogia , OpenTaps etc.

OpenBravo

Openbravo is an open source ERP solution designed specifically for the SME (small to midsize Enterprises). Developed in a web based environment.

TinyERP

Tiny ERP is a free ERP and CRM solution. The main technical features are: a GTK2 client and web interface, a distributed server,flexible workflows, an object database (on top of PostgreSQL), dynamic GUIs, customizable reports, SOAP and XML-RPC interfaces.

PostBooks

PostBooks is a full-featured, fully-integrated accounting, ERP, and CRM system, based on OpenMFG ERP Suite. Built with the open source PostgreSQL database, and the open source Qt framework for C++.

CK-ERP

CK-ERP is an open source accounting / MRP / ERP / CRM system that runs on top of multiple middlewares. It provides accounting and back office functionalities to SMEs and utilizes the underlying middleware to administer accounts/groups.