I like using selvage in my mail art. "Selvage" refers to the leftover material around stamps when you remove them from the sheet. I like to use the blank strips for addresses and the circular cut-outs of the new global international stamps make it into my designs on a regular basis. In modern sticker-stamps, selvage frequently has art or other information about the stamp printed on it. While you can find older issues with fancy selvage, it is more common to have the edges be blank or contain numbersor printers marks.
When I was a child and collecting stamps, the big thing was to have the "corner block," the section of a stamp sheet from the lower right corner of the sheet consisting of 4 to 6 stamps. That's because that block would contain the selvage area that held the plate number of the printing for that stamp. There's a great article at Linn's Stamp News all about the information you can learn from selvage. Over at U.S.P.S. stamps, there's a posting by Lloyd de Vries about the collectable value of plate blocks. Sometimes a stamp just isn't worth collecting without the selvage. Funny to think how something most people think of as trash can be so important to a collector.