SongKong 5.1 is the latest release - available from 16th of February 2018
SongKong is available for OSX, Windows and Linux.
The free trial allows you to try SongKong out on as many songs as you like in preview mode, you can see exactly what changes would be made but no changes are actually saved to your songs without purchasing a full license. To convert a free trial installation into a full installation you just have to purchase the correct license and enter the license details.
Detailed installation instructions are available from here