Queen Isabella of Spain (a/k/a Isabella I of Castille) and her husband Ferdinand II of Aragon are most famous for sponsoring Christopher Columbus on his plan to said to the Indies by sailing west (2000 miles, according to Columbus), which led to his "discovery" of America.
Columbus returned the next year and presented his findings to the monarchs, bringing natives and gold under a hero's welcome. Spain entered a Golden Age of exploration and colonization at this point.
They also brought stability to both kingdoms that became the basis for the unification of Spain. Later the two laid the foundation for the political unification of Spain under their grandson, Charles V, Holy Roman Emperor.