Partyhero - Pass your friends the aux!
MonitorControl - Control your external monitor brightness&volume on your Mac
Join My Playlist - Listen to music live together with your friends in Spotify.
Lunar.fyi - Control monitor brightness, adapt using the ambient light sensor, adjust volume, switch inputs and turn off displays without fiddling with clunky buttons.
OutLoud Social Jukebox - Control the music with your friends in real-time
Screens - GNU Screen.