Samsung didn't stray far from its comfort zone when designing the Galaxy S 4, and now a leaked build of Android 4.2.2 Jelly Bean for the S III makes it even harder to distinguish the older flagship from the newer one. The folks at SamMobile got their mitts on a test firmware build and, better yet, have combed through it to see what's new. As you may know, the S 4 ships with 4.2.2 under a TouchWiz layer, so it's not surprising to hear most of the features new to this S III build are on the S 4 already: an updated version of S Voice, more lock screen options / unlock effects, new display modes, a redesigned settings interface, voice control, and more. SamMobile has put together a video walkthrough of the build (embedded below), and you'll find an expanded changelog and software screenshots at the source link. Apparently, the firmware "works perfectly," so if you'd rather not wait through the (often lengthy) carrier approval process, you can download it for your S III right now (flashing required, of course).
Update: This is for the international model of the GS III (i9300).