Official Android 4.3 upgrades are currently few and far between, but you won't have to wait for one if you're willing to experiment -- the first CyanogenMod 10.2 nightly builds have arrived. As of this writing, those with Samsung's Captivate, Galaxy S Relay 4G or AT&T Galaxy S III can install the unofficial firmware to take advantage of 4.3's new features. Just be aware that you're taking more risks than usually exist with such firmware; this is an early release of unsanctioned code, after all. If that's no deterrent, the first releases of CyanogenMod 10.2 await at the source link.
Download: Get CM