Welcome to OpenKJ.org!
OpenKJ is open source karaoke hosting software licensed under the GPL version 3. It is comprised of KaraokeHost and BreakMusic, the purpose of which should be fairly obvious from their names. ;) It is currently considered unstable and itsn't 100% feature complete, but the primary developer is using it to host shows every weekend and it seems to be working fine with the exception of the occasional crash/segfault.
It should build and run fine on any modern Linux distribution as long as the required dependencies are installed (Qt5.x, GStreamer0.1x). You can follow the source code link to get to the project's GitHub page. This project is developed on Linux and then compiled for the other platforms, so the Linux version tends to be the most feature complete and to work the best.
Binary installers are available for Windows under the downloads link. Please excuse the giant installer size, I need to go through and prune down the gstreamer stuff to the parts actually needed by the program.
It will build on OS X, but many features are lacking because I can't get gstreamer to work on that platform. If any experienced Mac or gstreamer devs want to lend a hand, I'd be very appreciative.
I haven't had a chance to build the screen-shots gallery, but you can pop over to the GitHub page to see a few if you're interested.
Travis CI Status