Quoll Writer is free to use but currently only available for Windows. Quoll Writer is known to work on Windows XP, Vista and Windows 7, 8 and 10.
There is currently no version of Quoll Writer for Mac or Linux. To do either will take some changes, potentially large ones for Mac (Mac OSX is positively hostile towards tabbed interfaces) and Java look-n-feel implementations on Linux leave quite a lot to be desired. It is possible to do both but I'm only going to do so if there is a real demand, so speak up and let me know.
Quoll Writer is open source and made available under an Apache 2 license.
The code is hosted on Github.
I recommend checking out the readme on the Github page for details about the code and how to run it.