A Steam support page has accidentally let slip that Valve is planning tradable, virtual discount vouchers for use on Steam.
Valve has made no official announcements yet, but this support page outlines how the coupons will work. The coupons will be kept in the user's inventory, and can be traded with other users for games, other coupons or, indeed, hats. They can expire, and will be automatically removed from the user's inventory when they do so. They can be used on discounted games, but only one coupon can be used per item. It appears coupons can apply to multiple games - indicating that they could apply to entire publisher catalogues.
Personally, I've lost track of the number of times I've seen a sale on Steam, went to add money to my arse-tier, pre-paid credit card, only to find the sale has finished by the time the funds are available. So yeah, bring on the coupons.