I've used Symphony CMS (getsymphony.com) to build several websites, one of them quite large. I love it. It's the first CMS I've used that isn't a total mess architecturally. At it's core it provides a highly customizable interface for maintaining a database of content, XML data sources that pull from that database (or external XML sources), and XSL templates that are combined with the XML to produce the HTML pages of the site.

It's totally open source, and development is carried out on github (which includes all its extensions (http://symphonyextensions.com/)). The developer forums on the main website are active and many of the core developers answer questions rapidly. I can't recommend it enough.

Its downsides are 1) XSLT (some people can't stand it) 2) No polished off-the-shelf templates, you'll have to roll your own (but it's easy enough to get something up with Bootstrap and tweak it for your purposes).