Artistamps (aka Faux Postage) are small postage stamp like artworks which are made by artists to commemorate an event, express themselves on a social or political issue, and/or as products of their ImagiNations.  They can be printed or drawn directly onto an envelope or postcard or take the form of stickers or even something very much like official postage (dry gummed backs and perforated).  They can be produced as singles or in sheets, they can be one-offs or reproduced over and over again - anything an artist can conceive of and make real.  They are swapped, shared, and sometimes sold.  I have added artistamps to my mailings for decades and have swapped them through groups like Nervousness but have traded most often through a Yahoo Group called the Artistamp Mailing List.* Mail artists like Ryosuke Cohen and Ruud Janssen produce assemblages made up of submitted items, often including artistamps, which are then sent out to all who participate.  Mail Artist Laura Dunn hosts an ongoing Artistamp swap where you send in 11 artistamps and get back ten from ten different artists (she keeps one from everyone for her own collection).  Groups like the Carving Consortium and EraserCuts Exchange host First Day Cover swaps in which everyone carves a stamp (or stamps) which complement a U.S. postal stamp which is about to be released.  All who participate receive a first day cover which includes both an imprint of the carving and the postal service’s FDC imprint for that particular stamp. 

* Documentation: Diversity Docs (2002)  Day of the Dead Swap (2005)