I'm building Grow and the Grow SDK (http://growsdk.org) which has a lot of overlap with Statamic (file-based, content in Markdown, YAML things).

One of our tag lines is "develop everywhere, deploy anywhere" – as in, sites are file-based and can be developed locally or in the cloud on our web service. Grow can deploy to a variety of servers (S3, GCS, Dropbox, etc.).

The project's not quite ready for use yet but the SDK will be released as an open source project under the MIT License.

Someone submitted my introductory blog post about Grow on HN a few months ago: https://news.ycombinator.com/item?id=6562182 – Right now you can sign up for the mailing list, and when a developer preview is ready I'll send out updates. :) I hope to have something ready in a few weeks, actually.

http://getkirby.com/ is close, although doesn't seem as polished as Statamic

This seem to be commercial software as well. https://github.com/bastianallgeier/kirbycms/blob/master/lice...

Semantics aside about the source being available to read but not effectively use, that is...

http://jekyllrb.com/ maybe?

It is open source. You just have to pay for a licence to use it, but it's open source.

For clarity, Statamic is not open source. A license will get you the zip file full of code, and you can tinker with it as much as you'd like, but this is the actual license agreement: http://statamic.com/license-agreement