UltraStar Deluxe - A free and open source karaoke game inspired by the Singstar™ game, runs on Windows, Linux and macOS.
ChordIQ - Learn notes, chords, sight reading, perfect pitch and more.
KaraFun - KaraFun is the best online karaoke. Over 35,000 karaoke songs, with studio quality, at home or on the go. Regular updates. Free demo!
7Pad Scales and chords - Music instrument and assistant for playing scales and chords.
OpenKJ - Open source cross platform karaoke hosting software
muted.io - A magical collection of interactive music theory tools & visual reference pages for musicians and music producers