This project (geiss4pi) turns a Raspberry Pi into a plug-and-play music visualisation appliance: stream music to it over Bluetooth and watch on TV or projector psychedelic visuals that react to music, it outputs sound over HDMI and/or audio jack.
You can play with the keyboard interacting with audio visualisation.
The project (open source) is inspired by the legendary Geiss Winamp plugin (now open source).