ChordU - Extracts chords from any song, integrated YouTube.
Guitaa.com - Turn ANY song into chords, play along with interactive chords and diagram, transpose, loop, tempo control.
Chordify - Chordify turns any music or song (YouTube, Deezer, SoundCloud, MP3) into chords.
Riffstation - The web version is Free and you can learn how to play Chords with Youtube Songs.
UkeLib - Ukulele Chords - A comprehensive library of ukulele chords with suggested finger positions for every chord.
MyChord - MyChord is an application that allows you to play your favorite songs with both graphical piano keyboard and guitar tab display option, making you easily transcribe your music.