Tomahawk

Tomahawk is ''a music player that fundamentally changes the way music is consumed and shared. It decouples the name of the song from the source it was shared from - and fulfills the request using all of your available sources. This creates a universal translation layer across music repositories, streaming services and geographic territories. ''

This player is not so much good for playing your local music, but extremely good at playing external music. You can use places like Youtube, Soundcloud, Last.fm, Dilandau, Official.fm, Ex.fm, Jamendo, Vkontakte and 8tracks as resolvers (places to play music from). It gets better when you connect with friends through Jabber, GTalk or Twitter since you can then play their music as well.

It's available (free) for Windows, OS X, Linux. Currently the player is 0.6 beta so there still bugs and limited functioning.

Other Interesting Features

 * You can listen live to what your friends are playing.
 * Option to import your Last.fm-history (although it doesn't do anything specific with it right now).
 * It can import XSPF-playlists (the kind that Last.fm exports).
 * Option to export a playlist as an embed-file through toma.hk where the tracks are resolved according to the resolvers that you audience has access to.

Demo
These are the options as of the new 0.6 beta.