Using Retail Me Not, I've saved money on everything from bridal shower invitations to picture prints to custom postage stamps. If you can't find what you're looking for on Retail Me Not, you can sometimes find coupon codes on other sites through a regular Google search.
One bummer about Retail Me Not -- it seems that more and more companies are catching on and requesting that codes for their site be taken down, i.e. Target and J Crew. But even then, Target for example provides their widely advertised "featured discounts" so you might still find something useful.
Happy online shopping!