A few days back, Samsung announced that the Galaxy S III wouldn't be getting updated to KitKat, which naturally raised quite a few angry responses. Samsung touted memory issues as being the reason, but it's something the devs over at XDA weren't willing to accept. As a result, a few devs namely Kryten2k35, arter97, forumber2 and JustArchi, have gotten together to try and port over the KitKat ROM from the Galaxy Note II, and have actually succeeded in making it work. Apparently, the ROM was too big to fit on the S III's internal partition, but debloating the Note II ROM to fit on the NAND partition did the trick, and now Galaxy S III owners have an unofficial KitKat ROM to try out.
Unfortunately, the ROM is currently a work in progress, even though most things are working properly (things like sensors and calls have issues), which means you shouldn't try this out if you aren't too comfortable with flashing custom ROMs. The devs hope to get everything working properly using files from the KitKat update for the LTE model of the phone (GT-I9305), but for now, only the most courageous (or the most desperate for KitKat) should try this one out.
We'd like to note that while Samsung could have done the same as what the ROM's devs have done to fit the ROM on the S III, the company can't remove features and push out an official update, as that would be troublesome for users as some of them might be using those regularly. We wish they would provide the source files so devs can update on their own, but that's likely something they (or other companies will never do.
Head to the source link for all the details.