SongKong 3.7 is the latest release - available from 4th of January 2015
SongKong is available for OSX, from Lion onwards, and all flavours of 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 a license and enter the license details.