Mom has both a general collection of stamps from all over the world and a topical collection of music-related stamps, and she loves to get new ones. Whenever friends & relatives go traveling out of the country, she only wants stamps as souvenirs. :)
I also have a small (tiny) personal collection of origami-related stamps (see http://www.giladorigami.com/Articles-Stamps-Origami.html
for examples, although my collection has only a few of those stamps).
In addition, Mom collects stamps of all sorts (old/recent, used/mint, on paper/soaked off, US/non-US, etc.) for the Woman's Board of Missions for our state church conference and on their behalf regularly sends stamps to two charity projects, one in Germany ( http://www.briefmarken-bethel.de/start.html
), and another in the US inspired by the German program (Stamps for a Living/Deaf-Reach, based in Washington, D.C.).
She uses the priority mail flat-rate boxes and sends one out whenever she has packed one as full as possible - every few months. Both projects use the stamps to provide work and generate income for physically/mentally/socially challenged individuals.