Up until about two years ago during NaNoWriMo, I basically swore by OpenOffice and it's later update to LibreOffice. However, I actually had a major problem with it two years ago (possibly from an upgrade, though I only go for the ones that claim stability)...It kept eating my words! I'd work on a document, close it out, and then a few hours later I'd go to a write-in or something and...well, let's just say at one point I lost 10k of my manuscript. :/ That year, I stuck to Docs after the last straw (which was at least in the first week, conveniently).
Nowadays I'm using yet another freebie application, and so far I really love it. If you want a free/open-source program that has a fair number of extra bits like a way to keep notes, an idea board, etc...I can happily recommend Quoll Writer. It kind of reminds me of ywriter, but with a twist: It opens with a project manager, meaning you can get to all of your projects at any time from a simple interface, rather than having to open and close projects. It has a great word count and statistic tool that includes readability, and a distraction-free mode that takes up your entire screen (including task bar) if you so choose....which has a few nifty gadgets in and of itself.
That said, yWriter is another pretty nifty and free writing application you can use. In some ways I like it better in that for the object data you can keep, such as the character info and whatnot, it gives you a more organized outline for how to write about the characters and such. However, I'm not as big on the general UI and prefer Quoll for the project manager aspect. I think the best way to choose is just to try them and see what works for you...or rather, if they do!
I use a different program to write on the go however. I got it for free on Amazon Underground, but I'd probably buy it if I had to (if it were cheap enough) called LectureNotes. It's just another notebook app, but I tend to like them in general. I have about ten 'notebooks' right now in it, but I mostly use it for notes or general ideas for my stories as they come. I also use it for lists a lot, though to be honest typing can be a pain in it...