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.
Yokee - Karaoke to YouTube videos
7Pad Scales and chords - Music instrument and assistant for playing scales and chords.
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!
Scale Heaven - Browse and play any scale/chord, chord progression.