Used stamps - as in the type you get on letters and parcels - are seemingly useless and normally get thrown out, however, they can raise money for a number of charities.
Highstreet charity stores such as Marie Curie cancer care, Help the aged, RNIB and Oxfam to name a few will accept the stamps in the stores and make money from them.
I am not sure of the exact way they make money, but I'm pretty sure they sell them on to collectors. Even standard 1st and 2nd class stamps are gratefully received.
You need to make sure that you leave a margin at the side of the stamp so it does not get damaged.
There is a bit of info here: http://www.intelligentgiving.com/how_to_give/shift_your_junk/coins_stamps
This costs you nothing, takes no time at all and can make a difference. Spread the word!