Frequent firmware leaker SamMobile has just released what it believes is a build of the Android 4.2.1 update for the international (i9300) Samsung Galaxy SIII (S3). What we're looking at here is someone testing the new firmware -- Android 4.2.1 JOP40D -- that has chosen to give a system dump for others to try. It looks like many of the new features such as lockscreen widgets and Daydream are included, but this is certainly not final firmware.
SamMobile has re-packaged this firmware into an ODIN Flashable file that other users with the device can try out if they wish. There are of course risks involved, above and beyond the binary counter being increased and the device reporting as "modified" when running the firmware. If you're interested, you can give it a look at the source link below.