The Audioscrobbler system is a massive database that tracks listening habits and calculates relationships and recommendations based on the music people listen to.
After installing an Audioscrobbler plugin for your media player (eg iTunes, Winamp, Amarok) the name of every song you listen to is sent to the Audioscrobbler server and added to your music profile. The Audioscrobbler system powers our main site, Last.fm, as well as exposing data via webservices so other projects can make interesting things from the data and recommendations we provide.
Audioscrobbler was initially released by Richard Jones in 2002, with support for Winamp and XMMS. The protocol was publicly documented and support has been added to dozens of different media players and devices since then. To name a few: iTunes, Winamp, Windows Media Player, Foobar, iPod, Amarok, Rhythmbox, mpd, Xbox media center, Slimserver, Jinzora, mpg321, Muine, Rhapsody, YME, Soundbridge, VLC... and loads of other scripts and tools exist to scrobble and fetch data from our webservices.
Our official client supports the most popular media players. Please refer to the wiki to find stand-alone plugins and scripts to support other players and devices. If your player or device of choice for listening to music isn't supported, adding support is pretty easy. Refer to the developer page for some hints.