Ask HN: Is there an open source alternative to Travis CI

I'm wondering if there is an Open Source system, I can install on OSX, Windows and Linux, which hooks into a Git repository, so that I can automatically build software for all those platforms easily.

It would need to support a mark up language similar to what Travis CI uses, to enable dependencies to be downloaded prior to install etc.

The reason I ask this, is I've not found a single service which can build for all those platforms, whilst also being very cost effective.