Organizations and movements in conflict with a current regime have, through the years, issued many stamps. Sometimes the stamps have actually been used in areas under their control, but oftentimes they have been printed just for propaganda purposes.
Tamil Post (Sri Lanka)
The examples above are stamps that actually came into use. General Aguinaldo controlled some of the Phillippine Islands when he proclaimed the Republic Filipino 1899. He was finally arrested by American troops 1901 (the Americans already controlled the rest of the islands).
The Tamil post existed for a short time in 1961 in Jaffna in the north of Sri Lanka (an area controlled by the Tamil Tigers).