I've been using Storyist (Mac-only) for three or four years now, and I am completely and utterly in love with this software.
Right now, it's available as part of a bundle
containing a lot of other useful software (Aurora is a decent photo editor; Scapple is my mind map of choice, and a number of the rest of apps in the bundle look very yummy, too.)
Storyist works on the same principle as Scrivener (it also has an iOS app which I use frequently; though there's no Windows version if you need that); but for me the main difference is that I found Scrivener unintuitive and clunky: if you want to use it, you need to work through a lengthy tutorial and watching these five videos, and at the end of all that I still felt that I needed to mould my workflow to please the software rather than the other way round.
With Storyist, I felt inspired from the moment I first opened it, and it has improved my workflow tremendously.
I use the side-by-side feature extensively; and it's made a big difference to my writing: the fewer things I need to hold in my head, the more capacity I have for writing.
I like the ability to have the same document side by side, so you can keep writing while scrolling through and pulling up previous scenes for comparison. I've worked with two documents side by side when translating, but mostly, I will pull up relevant information in the second pane while never losing my place in the main one. For characters, I simply dump descriptions in there; but I may also keep reference photos, maps and diagrams. Recently I wrote a council scene where everybody was sitting around a table - having a seating plan made it possible to consider who would hear a whispered remark, who might inadvertently (or deliberately) step on someone else's toes, whether my protagonist would see a speaker's expression or not... it made for a much better scene.
I've also used Storyist for editing so I can keep characters and their quirks straight - this means that the descriptions of a technology remain coherent, but not identical. If a phrase is repeated too often, or all characters use the exact same phrase all of the time, readers will notice. At the same time, you want a certain consistency in how characters describe a new technology like mind speech or FTL travel: how does it feel, what metaphors do they use, does it _mean_ something if one character experiences it differently. And you don't want descriptions to differ between books, so you need to write them down _somewhere_... and I've found Storyist to be perfect for that.
As a bonus, you get outlining and notecard tools which are pretty good - I don't use them that often, but they're worth mentioning. This is a tool that will work in oh-so-many ways for very different workflows. I can't emphasise how much difference it has made to me to have all of my materials - different mss, notes, images - in one place, and to be able to access them without losing my place in the running text.
So if you haven't tried this kind of tool, now is a good time; $29 is as good a price as you're going to get (Storyist has a free trial!) and you'll get a bunch of other apps into the bargain.