The most well-known example was the Columbine High School shootings, an event that 'triggered' copycat shootings in the United States. The shooters were openly known as fans of Doom, among other things, with one of them claiming in a homemade video that the shotgun he held was similar to the one in-game. Since then, major school shootings usually become pinned on in some form on similar games, usually as empty speculation. In the aftermath of the Virgina Tech Massacre, infamous media crusader Jack Thompson attempted to link the events to Counter-Strike: Source despite, unlike Columbine, there being little-to-no proof that the killer had ever even played it or even games in general.
In recent years, the Grand Theft Auto series, mainly the installments III through IV, are blamed for murders of police officers and civilians. Some of the killers claimed during their eventual trials to have been 'inspired' by the games, in an attempt to gain the insanity plea, while others are shrugged off as speculations. Other games like Postal 2 and Manhunt are also pointed at.
Killer games: Murderers linked to consoles, Daily Star