Following the Samsung Galaxy S4 and the Galaxy Note 2, Android 4.3 firmware for the Samsung Galaxy S3 has also made its way online and is available for download, unofficially of course, courtesy of a SamMobile source.
This is the first Android 4.3 leak for the Galaxy S3, build version I9300XXUGMJ9, and while stable, is still a test build and will probably have a few bugs. According to SamMobile, some of the changes include a complete overhaul of the UI, making it similar to what can be seen on the Samsung Galaxy S4 and Galaxy Note 3, with the Settings app also featuring a new tabbed interface. Unfortunately, the original launcher and widgets are still being used, and Samsung Knox, which is an expected feature with the final release, is yet to be implemented in this version. Of course, since this is a test build, features and more UI changes may be available in future leaks and the final, official release.
You can download the file here, and find installation instructions from SamMobile here. SamMobile has tested this version and says that it is stable, but keep in mind that flashing custom ROMs and unofficial leaked firmware is not without its risks, and some amount of technical know-how is recommended. If you decide to go ahead, please follow the instructions exactly as stated. Also remember to back up your data before flashing, as installing this test build will wipe your device, including data on your internal SD card.
Let us know how it goes!