Since it looks like Pope Benedict's long-awaited social encyclical, Caritas in Veritate, will be appearing any day now (he reportedly signed it Monday), it's probably a good time to take a look at the history of Catholic social teaching, specifically as it has been expressed through papal encyclicals.
This encyclical tackles the turmoil surrounding labororers in the wake of the Industrial Revolution, touching on issues including socialism, unbridled capitalism, a living wage, the relationship between laborer and employer, and the relationship between classes. Pope Leo also mentions the preferential option for the poor.