Win/Mac: I use Scrivener (not free). I only use it for drafting - it is NOT meant to be a word processing piece of software. I get all Ragey McRageperson when people on the L&L forums ask why they can't do something word-processory. I like the ease of use (your mileage may vary) and make heavy use of the Synopsis and Document Notes features.
Win/Mac: The much maligned Word (or Open Office) could be used kinda like Scrivener, but you need to be disciplined about using proper headings (to make it easier to move stuff around e.g. like Scrivener's binder). I quite like Open Office. It's not as groovy looking as Word but does the same job.
Win/Android: There's also the "been around since forever" yWriter by Spacejock. I found it a bit overwhelming but people
swear by it. It's free!
There's also an er, outbreak of web based one. All seem to be subscription based (e.g.
http://novlr.org/).
I've tried most (all?) of the Mac ones but Scrivener is my favourite.
Before I mac-d, I tried yWriter and ended up using Word.