Society for the Promotion of Elfish Welfare

S.P.E.W. stands for the Society for the Promotion of Elfish Welfare. It was founded in 1994 by Hermione Granger in response to what she saw as gross injustice in the treatment of house-elves at the 1994 Quidditch World Cup.

Founding
Hermione thought it cruel that house-elves were neither paid nor given holidays for their work, so she campaigned tirelessly for a change in their working conditions. She managed to get several students to join, though they only did so to stop her from badgering them. These students included a reluctant Ron Weasley, who called the group "spew", and an indifferent Harry Potter.

Despite using various tactics, such as making badges and petitioning, Hermione's cause had very little effect. Eventually, she started knitting hats and socks, which she left lying around Gryffindor Tower, hoping to free some unsuspecting elf who picked them up while cleaning the common room.

House-Elves' Reaction
The majority of house-elves are accustomed to their work, and seem to enjoy it. They regarded Hermione's actions as insults to their race. Thus, all of them, aside from the radical Dobby, refused to clean the Gryffindor common room in protest. Since he was already free, Dobby took all the clothes and wore most of them; the rest he gave to fellow house-elf Winky, in the false hopes of cheering her up.

Impact
In spite of all this, there are indications that S.P.E.W. did eventually enjoy some moderate success, though it is likely that by then it was disbanded. Hogwarts' house-elves participated in the Battle of Hogwarts in 1998 in an unusually bold move for their kind, although this may have been a manifestation of their extreme loyalty to their "masters". However, it is possible that their contribution to the battle changed some people's views, as even Ron Weasley, previously and vocally critical of S.P.E.W., demonstrated concern for house-elves at the time.

Also, founder Hermione Granger went on to work for the Department for the Regulation and Control of Magical Creatures at the Ministry of Magic, where she continued to work for house-elf rights.

Also see
house-elves dobby kreacher