It's going to be a long time yet before devices like the Galaxy S III and Acer's Iconia Tab A700 get Jelly Bean 4.2, and the Samsung Captivate and Galaxy S II probably won't ever get another OS version update. That's why we love us some custom ROMs, and the CyanogenMod team has released nightlies of CM10.1 to all of these devices, including the MetroPCS and US Cellular variants of the GSIII.
It could oftentimes be unstable and not properly tested, lacking any changelogs, but eventually evolving into alphas, betas, release candidates, and finally stable releases.
You know the drill: while CyanogenMod releases are usually pretty stable, even in the nightly stage, flash at your own risk. Here's the flashable Gapps package for all CM builds.