Shared Spotify - Find the common songs you have between friends
Harken - Flashcard app using smart repetition to improve your memory
Spotify Codes - Share codes for songs, artists, and playlists from Spotify
Songlink - Share music with anyone.
Bands on a Budget - Economical SWAG meant for bands, often hijacked by startups
Lyrics Monster - Find fav songs from the part of the lyrics you remember