I am researching Enterprise Resource Planning systems, I found a very good definition on wikipedia at http://en.wikipedia.org/wiki/Enterprise_resource_planning
I found also a list of open source vendors:

• Adempiere
• Apache OFBiz
• Compiere
• ERP5
• GNU Enterprise
• SQL Ledger
• WebERP

Look also at: http://en.wikipedia.org/wiki/List_of_ERP_vendors

I am going to install the open source systems on my local machine and study them, I hope I can write posts about my experince with them.