Couldn't get it running on my ubuntu 14.04. npm keeps complaining about something or another, either with nodejs not being called node, or with gyp; at this point I kind of just stopped trying.
Something can be open-source and paid for. It is uncommon, because people can build it on their own, but Sublime Text is an obvious counter-argument: people are ready to pay for development, even when they could use it without paying.
There are build instructions for Linux and I think even Windows as well. I think Ubuntu is currently the main target on the linux side.
Edit: Just double checked and there are Windows build instructions at https://github.com/atom/atom. Although I haven't actually tried to follow them, so I'm not sure if they end in a usable build.
Just looking at the issue tracker seems to indicate there are some problems with the windows build, but it does seems to indicate they are well on their way.