I don't know about you, but my one of my pet peeves is making a purchase online and seeing a promo code or coupon code box and having nothing to input into it, knowing that I'm missing out on a saving opportunity.
Typically in this event, I'll google "Company X promo code" and see what I can get. One great consolidated resource is Retail Me Not. This site is the most comprehensive list I've found of current promo codes for a variety of retailers. You can search by store or just view the top deals of the day (currently 15% off at Kate Spade, 20% off at Old Navy, and 20% off at Bed, Bath, & Beyond just to name a few) or enter the retailer you're looking for in the search field. They have a list of deal options from a number of stores including the overall success rate by users.
There is a separate section for all free shipping deals, printable in store coupons, and printable grocery coupons that can be searched by category.
Next time you stumble across a promo code box, and have nothing to enter, give Retail Me Not a quick visit and see if you can get in on whatever deal opportunities are out there.