For me, I have an iriver MP3 player also which is used in the car, on the bike (via the Autocom) and in the gym. I used to have most of my music in Ogg Vorbis as well, but with the onset of the Xbox 360 I wasn't able to stream .ogg files to it and therefore had to convert all of my collection to MP3.
On the home PC I use WMP11 as it's fine for what I need it for, and with the integration between it and the 360 I've no reason to look elsewhere. I used to use WinAmp but I don't really tend to use this now because of the WMP11/360 integration. It's great being able to play a game on the Xbox and have your own music playing through the surround sound system at the same time (I know, albeit in stereo, but it's better than listening to the in game music!)! ;D
