First off, I will say that I feel slightly bad for posting this. Mainly because this is a very cool app, but the amount of people that will be able to use it will be limited. You see, this one is available only for those that are both using a Mac and have a jailbroken iPhone. Yep, just another reason why I feel like jailbreaking.
Anyway, the app is called Movement and comes courtesy of Mac developer Jeff Stieler. Just like the name implies you can easily move (re-arrange) the app layout on your iPhone, and by easily I mean you are able to re-arrange those apps on a display other than that of your iPhone, hence the required Mac.
Of course, this also happens to be one of the rumored iTunes 9 features, so everyone may be able to re-arrange their apps with ease soon enough. But in the meantime, Mac users can enjoy the ability a little earlier.