360 Live [App Store link] certainly isn't the first Xbox 360 app to hit the iPhone, but it's definitely one of the most full-featured, offering gamertag management, complete friends list access, messaging capability, games library browsing (with achievements and all), and even a CoverFlow-style friend browser just for good measure. It is pretty darn impressive -- if there's anything you need to do with your Xbox Live account (Gold account required for messaging) while away from your actual 360, this app will let you do it.
There is one issue: it's $1.99. Technically, that's not bad -- an app like this is probably worth the $2 especially if browsing your friends' games is something you do often. But the catch is that Microsoft recently stated that all apps that tag in to Xbox 360's online service must be sold for the low, low price of free. So one of two things will probably happen here -- either the app will drop down to free (in which case, you might as well save your $2), or Microsoft might end up blocking it, either on the App Store or in the actual Xbox Live connection. Hopefully, whatever happens, we'll end up with a solution