The goal of ERP isn't simply to provide a unified application interface for every aspect of a company's business process -- it also tries to include the inherent data-mining capabilities that go along with it. But while JD Edwards, Oracle, SAP, and others have invested significant resources in building ERP software that's customizable enough for any business, these companies have also forced the price of the software to heights many businesses just can't afford.
Enter open source.
A large part of ERP's high price tag goes toward the army of business process and programming consultants needed to customize commercial software for individual business operations. Access to open source ERP application code, on the other hand, means you can accomplish this customization in-house, while avoiding hefty software licensing fees to boot.
One of the best-known open source ERP apps comes from Compiere. With an eye toward business analysis, the software incorporates CRM, POS (point of sale), and inventory management modules that offer excellent flexibility for multiple business scenarios. The product integrates with most relational databases using JDBC, and it offers specific support for Oracle and Sybase. Server and client platforms include Linux, Solaris, and Windows, among others. The software is published under the Mozilla public license.
Available under the GNU license, webERP -- another production-grade project -- contains full accounting features, including general ledger, accounts payable, and accounts receivable books, as well as role-based security and a highly customizable Web-based front end. Oriented toward manufacturing-style business processes, it includes support for order entry and inventory management. As opposed to Compiere, it has no modules for customer management, HR, or similar resources. Gold, silver, bronze, and translation support are available from a number of companies around the globe.
Other projects are available, but one characteristic of open source is that different projects define their category's feature sets in different ways. This is especially true of ERP packages. Linux-Kontor, for example, defines ERP without accounting, focusing instead on customer management, order entry, invoicing, and inventory. TUTOS, on the other hand, calls itself ERP but more closely resembles a groupware suite. Clearly, some research is needed to make sure you're really getting what you expect in this category.