An old WinAmp for MP3's. Mainly because of the burning engine, they usualy go straight to CDR then into my car. I don't often play music while working on the computer, I have a TV right above my moniter.
For video, my default is this strange little bug that came with my ATI TV Wonder card. If it has trouble playing something, I go for MediaPlayer Classic. But the ATI program surprised me with its ease of use and general functionability, so I've stuck with it. For DVD's, usualy MPC, but if it's a DVD that I'm authoring and need to view from my HD (which is one of MPC's downsides), I use Intervideo WinDVD. This came with my old computer, and works just fine when I need it, despite being a resource hog.