"Citadel" is a send-off for Commander Shepard and her crew. The final DLC for Mass Effect 3 "offers an entirely different kind of closure for the trilogy" and, according to our review, is a "wholly welcome farewell" for the franchise.
While the DLC isn't free (it costs $15), BioWare is still giving away a gratis gift to fans. The "Citadel" soundtrack is free to download, including twenty new tracks for fans to listen to.
To get the soundtrack, all you have to do is visit the BioWare website, log in to Origin, and download the 300MB+ .zip file. (Update: You'll need to have registered your copy of ME3 beforehand, likely by playing the game while logged into Origin at least once.) The tracks are all DRM-free .wav files that should play in whatever media player you choose.