Mautic is an Open Source marketing automation software, which provides a whole bunch of functionality around contact tracking, campaign management, mass mailing, landing pages, and more. It can be self-hosted or used as Software-as-a-Service (SaaS). The source code is on GitHub and licensed under GPLv3.
It provides an API, and is already integrated with a whole lot of services, varying from social networks and instant messengers, to CMSes and CRMs. Scroll down the Tour page for a comparison table against such alternatives as Marketo, InfusionSoft, Hubspot, Pardot, and Eloqua.