Self-adhesive stamps were first issued in such tropical climates as Sierra Leone and Tonga in an attempt to avoid the sticking-together tendencies traditional water-activated stamps had in humid conditions. They also made die-cutting into fanciful and unique shapes easier.

The United States Postal Service's first foray into self-adhesive stamps was in 1974 with the 10-cent dove weathervane. Stamp collectors' criticism of the format in the U.S. began here, as the glue used tended to progressively yellow the stamps. Self-adhesive stamps have been criticized by stamp collectors as difficult to remove from covers, and to save as mint. Still, many individual citizens and small businesspeople welcomed the new stamps as more hygienic and less messy, and by 2000, virtually all new USPS stamps were self-adhesive.

Artistamps have also been issued in a self-adhesive format.

Reference: http://www.linns.com/howto/refresher/selfadhesives_20020218/refreshercourse.asp?uID=