TLDR Version: Head over to http://www.openkj.org to check out an awesome open source KJ application currently in beta. It is developed and maintained by a KJ who actually uses it for his shows.
I use Linux Mint as the OS on my primary laptop. While Linux Mint is very stable and fast, it has the drawbacks of many Linux operating systems, the biggest being that not all Windows applications run well on it. Specifically, a majority of the Karaoke hosting applications, including Karafun, don't work or work properly, even when running in the WINE engine that allows certain supported Windows programs to run.
For the past year or so, I've used an application called PyKaraoke in order to play Karaoke files for my family karaoke nights. Unfortunately, it is not a very full-featured application. It can play MP3 +G and MP3+G Zip files, along with some, but not all, video formats. It does not allow for pitch shifting of tracks and does have a basic queue system. The application is not consistent with how it presents the Karaoke lyrics on an external screen, sometimes the window has to be resized to fill the screen. PyKaraoke also hasn't been updated in many years.
I occasionally searched to see if there were any open source options and nothing came up, until last week.
There is a new Open Source KJ application which is currently in Beta and available for everyday use. It is called Open KJ and can be found at http://www.openkj.org . OpenKJ works on Linux, Mac, and Windows. The program is written and managed by an actual KJ who uses the program to host his shows. It has individual singer song queues, song rotation, pitch shifting with the ability to remember key settings for frequent singers. OpenKJ also has the option of intermission music and slideshows in between songs. The application also has the ability to display a ticker of the current and upcoming performers. In short, it is a full-featured hosting application. The developer even has a beta feature allowing performers to search and select tracks on a custom website address in real time! Did I mention that this program is FREE!?
That being said, the program is in beta status which means it may have bugs. I've run into some issues with populating my song database, including application crashes and having some issues with the way it parses out file names. That being said, the developer is extremely responsive and, considering the price (free) it is well worth taking a look into. The program is regularly being updated and if there are any programmers here, I'm sure the developer wouldn't mind some additional help.
Happy singing and KJing!
Update: The developer did some additional work on the database side of things so, the DB issues I had no longer exist in the latest version! Update came out yesterday evening. 
