I agree that KiCAD is the "true" open-source platform for making open-source hardware. However, it's still a bit of a novelty software package. Old versions are really stable, but lack basic features (push-n-shove routing, length tuning, etc). New versions have a ton of new functionality, but crash often.
Altium's CircuitMaker -- while itself not an open-source software package -- is built from the ground up for open-source hardware. It's completely free to download and use, and unlike Eagle's free version, you can do anything with your designs that you want (you're not restricted to noncommercial use, as free Eagle does).
It's cloud-based, so that means there's a huge library of shared parts ready to design with, and there's already hundreds of designs that you can download and contribute to.
Since it uses Altium Designer's codebase, it's obviously Windows-only (like most high-end CAD packages), but it runs well enough on virtual machines to be useful to people who run OS X or Linux.