OpenERP is open source ERP software you can download or use online via a paid subscription with a 30 day free trial. The installation is different to a typical Web based ERP and may create compatibility problems for Web site owners.
The new OpenERP version 6 is available at www.openerp.com. There is a good range of documentation including a downloadable book. You use PostgreSQL instead of MySQL and Python instead of PHP plus have the choice of a Web based approach or an OpenERP server. Most other aspects are typical of Web based applications.
OpenERP has hundreds of add-on modules for CRM, stock, project management, and everything else related to ERP. If OpenERP does not do exactly what you need, look through the add-on modules.
Would I use it?
The one reason I might choose to not use OpenERP is their use of Python instead of PHP. If I use ERP based on PHP and I decide to investigate the code or contribute an enhancement, I can do that with PHP. There is not other obvious argument against OpenERP. The documentation and range of features is well above average. You do not have the complexity of the Java based ERP applications.
Read the documentation. If you like the documentation and are not a programmer considering modifications, or are a Python programmer, test the product with the demonstration data.