First up: mod tool OpenIV has been updated to support Grand Theft Auto V. GTAV is similarly devoid of official modding support, but it seems likely that people will again prove undeterred.
Despite no official tools to support it, Grand Theft Auto 4 had one of the best modding scenes around, adding new cars and improved textures, rocket punches and gravity guns, letting you play as a police officer or Superman, and much more.