According to a new FAQ , Steam now supports coupons. These tickets will sit in your inventory, and can be traded. Each coupon can be used on one game, but it doesn't look as though they'll operate as general Steam gift vouchers. The FAQ mentions that each one can be tailored to offer discounts on a certain selection of games. When you buy a game with a relevant coupon in your inventory, Steam will automatically apply the coupon discount. From there, you can choose to remove the coupon and save it for later.
This will allow developers and publishers on Steam to offer discounts on specific chunks of their catalogue to specific customers. It's easy to imagine coupons being offered as prizes, operating as rewards for loyal customers or high ranking community members. THQ, for example, could offer a bonus 20% off coupon to customers who have bought three games in their catalogue. The FAQ says that it'll be possible to redeem coupons on already discounted games "unless specified otherwise," which could lead to some spectacular savings when used in combination with one of Steam's mega-sales.
Valve haven't officially announced the coupon system yet. The FAQ is the only indication that they've been added, but the timing is good. It arrives ahead of the seemingly inevitable Steam Christmas Sale.