Here's a list of currently available AR, or AR variant rifles, that use some kind of gas piston system that tries to improve on the original AR gas impingement system. No doubt there are more than what I've listed here, but these are the ones I know about. Please add more if you know them. I've listed both clones, and AR "variants" (meaning rifles whose design significantly copies aspects of the AR). Note that several smaller companies that just assemble AR's (but don't manufacture them) are taking some of the components from companies below (such as POF), and creating their own gas piston rifles with their own name label on them.
CURRENT GAS PISTON OPTIONS
SigArms: The Sig 556
Armalite: The Armalite AR-180B
. A quality, classy AR-like rifle that predated the modern AR, and that used a good gas system long before the AR became popular. Still available, and often overlooked today.
Robinson Arms: Robinson XCR
. A variant that has some interesting, rapid barrel and caliber-change features.
Patriot Ordnance Factory(POF): P415 gas piston rifles and uppers.
ARES Defense: GSR-35 Black Lightning kit
(not a rifle but a drop-in gas piston conversion system for existing AR's)
Bushmaster: Gas piston rifle
Land Warfare Resource Corp (LWRC): 5.56 Gas piston rifle
. Actually, not sure you can get one of these yet. Defense Reviews says these might be "an alternative to the HK 416 for civilians."
FUTURE GAS PISTON OPTIONS
Other companies that either have one we cannot get yet, or that supposedly have one coming soon:
H&K: The 416
that we've been talking about. Deployed in service units, but either hard or impossible for civvies to get one. This Defense Review report
gives a LOT of info about the status of the HK 416 and related alternatives.
FN: The SCAR
. Deployed in service units, but acc. to Impact Guns, civilian versions have not been announced yet.
Lewis Machine Tool: Supposedly bringing out a version on their cool Monolithic Rail Platform
AR's. This would be my type choice, in a real-world, affordable, high-quality upper, because their MRP builds are awesome quality. Supposedly due out this December.
Colt: Supposedly already has prototype gas piston rifles, with plans to eventually test in the military.