OpenKJ is cross-platform open source karaoke hosting software licensed under the GPL version 3. It is developed by a professional karaoke DJ who was unhappy with the lack of good karaoke hosting software available for Linux.
It is developed on Linux (fedora, specifically) and built and tested on Linux, MacOS, and Windows. In theory it should build fine on the BSDs as well, but nobody has verified that.
It also now features integration with songbook.openkj.org for web based search and song submission.