Earlier we have posted about Leaked Android 4.3 test firmware for Galaxy S4 (GT-I9505) now you can download Android 4.3 for Samsung Galaxy S4 - I9500. You can upgrade it using Kies (if it is released for your country).
Or you can download and flash it using Odin. I just upgraded my S4.
Here is what is new
TRIM Support (Makes the device much faster)
Samsung KNOX Implementation (KNOX bootloader and dedicated application)
Samsung Wallet comes pre-loaded
Improved RAM management
Improved Display color reproduction (Display looks much sharper than before)
Improved TouchWiz Launcher (Much less launcher redraws and less lag)
New Samsung Keyboard
New Samsung Browser (Full screen by default, new tab interface and more)
New Reading Mode (Optimizes display for reading, used by only a few specific applications)
New Camera firmware
Minor UI tweaks (Contacts app, Flashlight Widget, dialog boxes etc.)
Check this post for 4.3 firmwares for Samsung galaxy S4 I9500on XDA