FrontAccounting is a simple, but powerful, system for the
entire ERP chain and covers:

Purchase Orders
Goods Receivable Notes
Supplier Invoices/Credit Notes
Payments
Allocations
Accounts Payable
Items and Inventory
Stock
Manufacturing
Sales Orders
Customer Invoices/Credit Notes
Deposits
Allocations
Accounts Receivable
Among many others...

FrontAccounting is free and released under the GNU General
Public License.