Odoo is the world's easiest all-in-one management software. It includes hundreds of business apps:
CRM | e-Commerce | Accounting | Inventory | PoS | Project management | MRP | etc.
In older versions there has been the caldav and crm_caldav modules to synchronize your calendar with most mail clients via WebDAV.
Those modules can still be found in the openerp extra addons branch, where the community tries to port it to v7:
I, myself, try to use these modules to synchonize ERP with Thunderbird (with some success).