Mapp v. Ohio (1961)
After police forcibly entered the home of Dolree Mapp in search of a fugitive and then searched her entire home without a search warrant, she was arrested and charged with possessing illegal obscene (pornographic) materials.  The US Supreme Court ruled that evidence seized without a properly executed search warrant had to be excluded from state as well as federal courts.  This is known as the "exclusionary rule."

http://www.oyez.org/cases/1960-1969/1960/1960_236/