Green Man Gaming (GMG) has a new feature to their site where you can earn either Cashback or Creditback on certain game titles. The first game they are promoting with this feature is the new X-COM: Enemy Unknown (Steamworks).
The way it works is when you purchase X-COM, you can choose to earn $5 cashback on your purchase, which is basically saying $5 off. Alternatively, you can choose to get $10 creditback, which is credit you can use on a future purchase at the GMG store.
Unfortunately, it looks like games that have this cashback/creditback offer will not stack with any voucher codes. At least not with X-COM.
You can read more about their cashback/creditback feature on their official blog post here.