As one of the transcendentalists who emphasized individualistsic ideals, Henry David Thoreau contributed to ethics the idea that it is one's rightful responsibility peacefully to protest what seems wrong in government. His essay on the subject, On Civil Disobedience, was written in response to his spending a night in jail for not paying his poll tax. The money collected would go to support the Mexican War, which Thoreau thought was wrong and unjustified, so he refused to pay.
Perhaps Thoreau's greatest impact on ethics lies in the influence his ideas had on later non-violent protestors. His idea of nonviolence combined with firm refusal to go along with what is wrong inspired later leaders of reform movements such as the civil rights movement in America.
On Civil Disobedience