Spotify - Map shows when two people play same song at same time
Virtual Audio Cable - Ever wanted to record your speaker output? (loopback) This is for you.
SoundCloud - Enjoy music & follow favourite artists
Loopback by RogueAmoeba - Get all the power of a high-end studio mixing board, right inside your Mac!
Ampache - Web ( HTTP centralized ) based audio/video streaming application
JACK Audio Connection Kit - JACK Audio Connection Kit|Home