Enterprise resource planning (ERP) is business management software—usually a suite of integrated applications—that a company can use to store and manage data from every stage of business, including:

• Product planning, cost and development
• Manufacturing
• Marketing and sales
• Inventory management
• Shipping and payment etc

[Ref: http://en.wikipedia.org/wiki/Enterprise_resource_planning]

List of ERP software packages (Free and open-source)

Name	Platform Technology
Adaxa Suite	Java
Dolibarr	PHP, MySQL
Fedena	Ruby, MySQL
OpenERP	Python, Javascript,PostgreSQL
...	…

[Ref: http://en.wikipedia.org/wiki/List_of_ERP_software_packages]

What is OpenERP(www.openerp.com)?

 

OpenERP is a modern Suite of Business Applications, released under the AGPL license, and featuring CRM, HR, Sales, Accounting, Manufacturing, Warehouse Management, Project Management, and more.
It is based on a modular, scalable, and intuitive Rapid Application Development (RAD) framework written in Python.

Some built-in applications(modules):

CRM-Customer Relationship Management(https://demo1.openerp.com/#view_type=kanban&model=crm.lead&action=404)

[Ref Img: from https://www.openerp.com/apps/crm/]

Point of Sale(https://demo1.openerp.com)

[Ref Img: from https://www.openerp.com/apps/point_of_sale/]

Project Management(https://demo1.openerp.com/#view_type=kanban&model=project.project&action=227)

[Ref Img: from https://www.openerp.com/apps/project/]
Accounting(https://demo1.openerp.com/#page=0&limit=80&view_type=list&model=account.invoice&action=353)

[Ref Img: from https://www.openerp.com/apps/account/]

Human Resources
Warehouse
Sales
Manufacturing
Purchase

and many more modules you could get in use. And even modify them according to your need.

Enjoy!!!