I've heard of Chromium, just as I have heard of Darwin.

Regardless, the proportion of users who use these technologies is a vanishingly small fraction of the Chrome and OS X marketshare.

but at least an open source version does exist. IIRC, the reason there is an open and non-open source version of chromium/chrome is that chrome contains some proprietary 3rd party software that cannot be open sourced.

>but at least an open source version does exist.

So what? If the goal is to promote the users' freedom, what use is it to develop software they do not use?

Chromium is used quite frequently on the Linux side of things. Distros like Arch Linux and Debian distribute only Chromium in their main repos.