The iPhone 5-ready apps are few and far between: here's a list of some
The iPhone 5 has a new resolution and what Apple did to avoid the older apps, written for 640x960 pixels is that it just letterboxed them with two black stripes on the sides, so it's as if you have a 3.5" phone again.
Apple's own apps, however, have all been updated to take advantage of the new resolution and the extra row available for info and icons now. The updated stock apps are iWork suite (Pages, Numbers and Keynote), iLife suite (GarageBand, iPhoto, iMovie), iTunes U, Cards, Find My iPhone, Find My Friends, Podcasts, Remote, iBooks and iTunes Movie Trailers. These will work like a charm on your iPhone 5 with iOS 6.
There are a few popular third party apps, which have also been updated to reflect the new fragmented iPhone realities, and these include stalwarts like Twitter, Instapaper, Tweetbot, Reeder, Evernote, CNN, Shazam, NYTimes and the popular games TempleRun and The Dark Knight Rises.
These are all must-have applications, but there are hundreds done with the 4" screen update now. While it is cool to have extra info on the same screen in the case of newspaper or social networking apps or extra items like a rear view mirror in games, because of the larger screen, it will take a while until all popular apps get updated for the new resolution.