There are two giveaways that I've found, and the first one has been around for a while, so you probably have already heard of it. But if not, here's one: Back to School Giveaway Bash at Reading Teen! In the giveaway, there are a bunch of books and a lot of ARCs. The giveaway ends September 14th, so hurry up and head on over there :)
There's also one at Shut Up! I'm Reading called a Contest of Epic Proportions, which I'm guessing is pretty BIG! There are tons of books on the list that you can win, and they might be adding more :D This one ends on October 17th, so you still have a while to sign up for it.