I used ofbiz for some projects, a joyful experience. It's now under the apache umbrella: http://ofbiz.apache.org/
From the website:
The Apache Open For Business Project
is an open source enterprise
automation software project licensed
under the Apache License Version 2.0.
By open source enterprise automation
we mean: Open Source ERP, Open Source
CRM, Open Source E-Business /
E-Commerce, Open Source SCM, Open
Source MRP, Open Source CMMS/EAM, and
so on
I used it to build an ecommerce application to sell customized products to consumers. I used the webshop part, the production planning and warehouse management.
Beware that it takes some time to dig into this huge framework but depending on your actual needs it will be worth it. There is also decent commercial support by a lot of service providers.