UltraStar Deluxe - A free and open source karaoke game inspired by the Singstar™ game, runs on Windows, Linux and macOS.
KJams - KJams is a free app offering the largest selection of karaoke tracks in one easy-to-use place.
Karaoke Mugen - Video & Movies and Audio & Music
OpenKJ - Open source cross platform karaoke hosting software
TunePrompter - TunePrompter is a karaoke player for your smartphone that is making it easier to make your own Karaoke videos.
KaraokeParty.com - Sing karaoke online and get a score based on your performance.