Alex was asking about the classic "Civil Rights":http://folkmusic.about.com/od/toptens/tp/CivilRightsSong.htm songs and of course _We Shall Overcome_ comes to mind. I didn't realize that it was actually a relatively recent song that Pete Seeger adapted from a traditional gospel song and was originally from a melody called "No More Auction Block for Me" and the famous version is by Joan Baez from 1963.
Some other classes are Blowin' In The Wind by Bob Dylan which is still wonderful.
And it brings up "Protest Songs":http://folkmusic.about.com/od/toptens/tp/Top10Protest.htm in general like This Land is your Land
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