Clip and save your cancelled postage stamps. The stamps are then sent to children's organizations and children's hospitals for craft projects. This group also sends stamps to schools for children interested in stamp collecting or to the art departments of schools for craft projects.
Hint: Tape the address for Stamps For Children on the outside of an extra coffee mug you have and keep the mug on your desk to collect stamps you clip and save. When the mug is full, send the stamps off --- Don't try to save hundreds or thousands.
UPDATED INFO as of 8/18/08
Please note that after 20 years and 20 million stamps, Stamps for Children has joined with Stamp Camp USA. All future stamp donations should be sent to the following address:
Stamp Camp USA, 117 Court Street Suite A,
The email is email@example.com.