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. :-)
