Your media player.

Windows Media Player 11 (it’s worth having because it’s a licensed version), Winamp (I used to use it exclusively), AIMP2 (now I’ve switched to this program – it’s convenient for working on the computer, it’s compact, and it has a pop-up menu for switching between songs – it’s quite nice overall), jetAudio (for some movie formats), and a bunch of other things... :D I also have a fun program called USSR installed on MKVFile, which has a lot of applications for different video formats :D