Spotify is C++ using the Qt4 toolkit. At least on the Linux client (still in "preview"), it also uses DBus to allow external applications to send command signals and receive current song information from the client. I actually wrote a wrapper for Spotify to translate Gnome media key signals to the MPRIS standard that Spotify listens for.

http://noswap.com/projects/spotify-gnome http://github.com/jreese/spotify-gnome