The best advice is to try lots of the options as tool choice is a very personal thing.
I've settled on
Novlr, with my outlines and notes held in
Workflowy. I love the way the interface fades out of sight while writing in Novlr and I'm just an Alt-Tab away from my outline. I like Scrivener, but the Novlr / Workflowy combination is much better for moving from machine to machine with everything safely stored in the cloud. Both work offline fine too.
But Novlr isn't free, of course, which was one of your hopes.
For free, give Google Docs a try, with Workflowy for outlining. This will allow you to move between your Mac and your Windows laptop seamlessly, and Google Docs has excellent version control and a good commenting feature for editing.
Graham