Part of the beauty of Android is that you're not stuck using the music play that comes with your phone. There are many, many music players to choose from in the Android Market.
I think this is a little different. There's a huge advantage to developing a common ecosystem around a programming language. For a music player application... hmm not so much.
Pretty much whatever you want (other than iTunes of course). Basically anything that can sync files to a USB drive.
As a longtime iTunes/iPod user :-) I've been using doubletwist to sync some of my existing iTunes playlists to my Nexus One. If you're not an existing iTunes user, something like SongBird (http://www.getsongbird.com/) might be a good choice. I think Windows Media Player will copy playlists to a USB drive (not certain about that).
Another alternative for iTunes users might be iSyncr (http://www.appbrain.com/app/com.jrtstudio.iSyncr), which looks pretty interesting. I haven't tried it yet, but supposedly it tricks iTunes into thinking your Android phone is an iPod. :-)
Nice, now if only Android could make a decent music player...