Gregg v. Georgia (1976): The court ended the moratorium on the death penalty by affirming Georgia's solution to the earlier Furman decision, in changing how its juries could impose the death penalty (specifically on one convicted man, Troy Leon Gregg). The court's holding set forth two conditions of a constitutional death penalty: "If the jury is furnished with standards to direct and limit the sentencing discretion, and the jury's decision is subjected to meaningful appellate review."
(A third case related to the death penalty also began in Georgia: Coker v. Georgia, decided in 1977, ruled that the Constitution did not allow rape to be a crime eligible for the death penalty.)
Heart of Atlanta Motel Inc. v. United States (1964): The court upheld Congress' authority, through the landmark Civil Rights Act, to "prohibit racial discrimination in public accommodations," which critics had argued was an intrusion of federal authority into private businesses. (The motel named in the case had denied black people from renting its rooms.)
Rome v. United States (1980): The court upheld Congress' authority under a provision of the Voting Rights Act of 1965, which required select locations nationwide with a history of discrimination to get federal approval before making voting changes, to block voting changes on the basis of their racially discriminatory effects — even if the changes were made without the goal of racial discrimination.
(In a 2013 decision, the court would strike down the formula used by the Voting Rights Act to select the locations which needed federal approval.)
Stanley v. Georgia (1969): The court affirmed the First Amendment's protection of private possession of obscene/pornographic material — a decision which further crystallized the Constitution's implied right to privacy from government action (especially as regards one's actions in a private residence).
Worcester v. Georgia (1832): The court's ruling is less important for its immediate effect, which overturned a conviction, than for its foundation as legal precedent in describing tribal sovereignty in the U.S. "States have no criminal jurisdiction in Indian Country," the court held (and thus the law the man broke was invalid), because the federal government has sole authority in dealing with Native Americans, as it would in dealing with other nations.
For further reading, consider this list of 15 cases highlighted by the University of Georgia's law magazine.