Windows Media, for the most part. It's a love/hate relationship because it's easy to use, and it categorizes songs under albums, artists, genres, playlists, etc; but on the other hand, mine, for some reason, usually cuts out the first second or two. Even if it was constantly exactly 1.5 seconds (for example), it wouldn't be so bad, but it doesn't - sometimes it might cut out two seconds, sometimes it might be half a second. The fact that it's random really irritates me.
My PC kept using WinAmp as the default music player, and in the end I got sick of it and deleted WinAmp

I also like using iTunes, because it does much the same thing as Windows Media, but is generally better, faster, etc. I don't have it on my computer though, and I don't have an iPod either.