In the past few decades, in order to raise revenue, postal administration of various countries have issued a growing number of .... scented stamps!

This one to the left is the coffee-scented stamp by the Brazilian postal authorities. Some weird ones include a sweet-and-sour pork scented stamp from China, and burnt-wood stamp (to raise awareness of forest fires) from Brazil.

Stamps of Distinction blog has more on the World's Smelliest Postage Stamps: Link - Thanks Tony Servies!