QuollWriter is similar, free and written in Java (open source) so should be portable. I have used it only in Windows, but find it quite servicable.
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Quoll Writer is an open-source and quite well-designed (in my opinion) program with an extensive feature-set.
It's free, has story/character/item management tools that you can use (or not) as you please, cork-boards, readability indices, customizable writing analysis, solid import/export, logging, encryption, even achievements. All of which can be used or ignored at your discretion.
Windows-only at the moment. I think it's a nice alternative (note I didn't say "better" or "worse") to programs like Scrivener.
Oh, and FocusWriter is quite good. Sometimes you just want to be alone with the page, and massive feature-sets might be too distracting.
Quoll Writer is good (Windows only). It seems to hit many of the most popular features of Scrivener without being overcomplicated. Open source.
There's also Manuskript and SmartEdit Writer. All free.
Actually a bunch of stuff out there, it really depends on what features you need. The Scrivener 3 release candidate (beta) is looking a lot better than Scrivener 1, too. It's good to have choices!
May I suggest Quoll. I have been getting some serious work done thanks to this software. It's in active development and open source as well. Super clean interface, entirely distraction free and the warm up mode is worth giving a try.
Your OS is Windows, I assume? I use Scrivener, but I've also been playing around with Quoll Writer and I like it. If I wasn't 40k words into a story in Scrivener, I'd likely be using Quoll Writer exclusively, in no small part because it's open source, free, and the creation of a redditor.
I'd say Scrivener likely gives you more organization options, but to be honest, I barely use them at this point, as my current works have only a few characters/locations.
I tried yWriter, Celtx, and Storybook (which seems to have dropped off the face of the earth) but didn't find any of them worked as well for what I wanted.
Edit: comma
You could try my (shameless plug) writing application Quoll Writer. Alas it only works on Windows but if you like it, and are prepared to tell others about how great you think it is, then you can have a free license. Try it and if you like it send me a personal message with your email address and name and I'll create you a license.
Quoll is a pretty good free alternative to Scrivener, and the creator posts here sometimes I believe.
I don't know if it has ALL the features of Scrivener, but I like the very clean UI and the full screen mode is nice.
Quoll writer has a distraction-free mode. It's worth checking out. It's not quite as versatile as Scrivener, but all the basic functionality is there. The interface is also very user-friendly, much more so than ywriter and scrivener.
Shameless plug for my own software: http://quollwriter.com
It only works on Windows at the moment (a Mac version is coming).
More information: http://www.reddit.com/r/writing/comments/fv1qw/writers_of_reddit_let_me_introduce_you_to_quoll/
Give it a try, if you like it you can have a free license, just send me a private message with your name/email address.
Full screen mode, color selections for the font and background and support for bold/italic/underline are coming in version 1.4.1 which I'm hoping to finish in 2-3 weeks.
Everyones advice here is sound, you don't need anything fancy. Sometimes you might want something that helps with formatting, keeps extra notes on hand for quick referencing, or lays out scenes in an easier way to visualise/rearrange at a glance. By notes I mean things like world building definitions or notes to yourself. If you're hunting through notes for half an hour trying to find info you want to reference quickly, then you're not writing either. So long as you dont let yourself linger over adding every minor thing, then finding notes or scenes in order in a sidebar can be a huge help.
Personally I really like either Quoll Writer, or SmartEditWriter (both free) for keeping things together, formatting, editing down into a script I like to work from. Neither are available for mac. If its only a preference but not a necessity then they might still be worth mentioning. I've found them to be the most efficient for me and have everything I need, but everyones different.
I'm not /u/everydayover, but if you like to write to the sound of a typewriter, Quoll Writer has an option to play a typewriter sound every time you press a key. Personally, I find it very relaxing.
Shameless plug for my writing application: Quoll Writer
It's free and open source and if you combine it with DropBox you get cloud saving and a better option than Word for free.
Not too long ago there was some mention of Quoll Writer which looks interesting, but I've only messed with it in practice. If you don't necessarily need it integrated into the word processor then LitLift looks interesting.