OpenKJ is cross-platform open source karaoke hosting software licensed under the GPL version 3. It is currently considered roughly beta quality and isn't 100% feature complete (it is getting really close), but all of the features required to run karaoke are working and the primary developer is using it to host 5 hour shows 2 nights a week with rotations of roughly 25 singers on average and is having no problems.
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.