Mautic is currently the only fully-featured, omnichannel Open Source solution for marketing automation on the market. From a feature perspective, Mautic does not have to hide from the big, proprietary players.
Mautic was founded in 2014 in Boston and is similarly organized as many other Open Source projects. There is a free community version that can be downloaded and self-hosted at https://www.mautic.org/. There is also a commercial version for enterprise use, which can be rented from the cloud as a SaaS solution.