If you want to have your very own SAP or Oracle suite of applications, I can recommend
OfBiz. It does need a fair bit of reading of the support web sites to get it running (well, it did for me) so it's not for the feint hearted or IT challenged.
It covers most business requirements from CRM through stock control, wages and web portals to name but a few.
Lots of material on-line as well for tutorials and documentation.
ST