Tinkerers over at XDA have just found found a leaked Jelly Bean build for the Galaxy S3, which is rumored to be officially rolled out at the end of August. While it may not be the final build, it looks pretty damn stable, and it brings the 19300 all the latest Jelly Bean features, "including the revamped notification bar and Google Now," according to XDA.
You can download and flash the build by following the instructions in the XDA thread here, but you better be running a stock Samsung ROM on your device or you run the risk of bricking your phone. Also, in order to run the new ROM, you better be well-versed in Odin and aware of the risks of flashing an unofficial ROM. Follow the instructions word-by-word and be careful.
That said, this build is legit. It integrates all the Jelly Bean features with Samsung's Touchwiz Nature UX (which may actually be a bad thing for some folks, but considering how many Galaxy S3s have been sold, someone out there must like the UI). For those who need proof, here's a Galaxy S3 running the unofficial ROM:
If you're unsure of flashing this ROM, you can always wait for the official update to begin rolling-out around August 29th!