Edit - the map below no longer works try this GTA V Map instead.
Grand Theft Auto Google Map
I wondered how long it would be until a Google Map of GTA IV would appear. The answer was not very long, with IGN being the first off the mark with this user contributed map.
IGN have created image tiles from the static GTA IV Liberty City map to create an interactive map using the Google Maps API. The map includes a large number of selectable layers organised into different categories; Points Of Interest, Restaurants, Shopping, Services, Entertainment and Nightlife, Emergency Services, Missions, Items, Vehicles, Secrets, Multiplayer and NPCs.
Registered users of the site are able to add markers to the map by right clicking on the map and logging in. This should mean that map will become more and more detailed over time.
So if you are searching for something in this huge new game IGN probably has it mapped.