Well, I think there are more options for ARs than there are people on the planet, so you're golden there. Notable brands are Tokyo Marui, Classic Army, G&P, King Arms... I'm sure there's plenty of people who will attest to others, but those are some big ones.
G3 on the other hand, I can only recommend. I love mine, and there's a hell of a lot fewer of them floating about when compared to Armalites if you're into being a bit different. Don't quote me on this, but I think Classic Army is your best bet if you want metal. TM makes them too, and I'm sure they're solid performers, but I didn't buy a metre-long rifle to have it made out of plastic. JG and Echo1 also make them I believe.
I have the CA G3 SG/1 and I love it to death.