The Warren Court's infamous Brown v. Board of Education decision, stands as one of the premier examples of judicial usurpation in the past century. Regardless of the nobility of the court's intention (renouncing the doctrine of "separate but equal"), there can be no doubt that the court exercised power that it had no constitutional right to exercise.
Pat Buchanan writes as if Brown was the first time the Supremes had trodden upon state prerogatives, even though I am certain he doesn't really believe this. He does, however, give a useful list of ways the court has stepped out of bounds in the 50 years since the Brown decision was handed down.
Even if this wasn't the first usurpation, George Will notes that the Brown decision marked a significant paradigm shift in the court's reasoning (emphasis added):
Brown v. Board of Education accelerated the process of bringing this creedal nation into closer conformity to its creed. But the decision also encouraged the abandonment of constitutional reasoning -- of constitutional law. It invested the judiciary with a prestige that begot arrogance. And it seemed to legitimize a legislative mentality among judges wielding an anti-constitutional premise. The premise is that ``unjust'' and ``unconstitutional'' are synonyms.
It is not difficult to see this theme permeating the decisions of the court in the half century that has followed.