With OpenObject, you may enjoy building on top of OpenERP existing core modules:
crm, help-desk, production, project, event, stock , human-resources accounting … but you are not obliged to.
One marvellous feature of OpenObject is the capability of a module to bring modification to existing objects and views. It is really a developer equivalent of the lancet used in surgery.
You can also enjoy re-using thousands of modules available on http://apps.openerp.com.