Quoll Writer is currently only available for Windows. Quoll Writer is known to work on Windows XP, Vista and Windows 7,8 and 10.
Quoll Writer is completely free as of version 2. If you already have a license then don't worry, this version will work fine with or without it.
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 bit 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.
As of version 2 Quoll Writer is open source (Apache 2 license).
The code is hosted on github and can be found at: https://github.com/garybentley/quollwriter.git
I would also recommend checking out the github page for the readme which has details about the code and how to run it.