A new, free little Windows app by Matthew Allen called PhoneAble does exactly one thing for iPhone owners: it enables them to manage the music on their device from any Windows computer with iTunes installed.
It does this by changing the computer's iTunes key--which is unique to each installation of iTunes--to match the key of your iPhone. This tricks iTunes into thinking that the phone plugged in via USB is one it's allowed to edit. It's simple, and that's why it's good.
Above is a screenshot showing how easy it is to use: back up the iPhone using iTunes, set the keys to match, and check to see if its working.
Sadly, there's no Mac version, and we're not sure whether there is going to be one, so don't hold your breath. But Windows users might want to keep a copy on a thumb drive in their pocket for when they're on the go.