Used postage stamps are accepted by some charities as a fundraising tool. They sell the stamps in bulk to collectors and stamp dealers, who in turn sift through them to find valuable or rare editions.
As of this writing in 2014, Sons of Norway in Las Vegas, Nevada, is one charity that accepts used stamp donations. Be sure to read each charity's guidelines before donating your used stamps. Most organizations require the stamps to be clipped to certain specifications, and they do not accept damaged stamps. Because the stamps are sold to dealers, commemorative envelopes and stamp collections are especially welcomed and appreciated.