IGN utilized Google Map's engine to create a huge, zoomable map of Liberty City. It comes complete with various Points of Interests and other useful information. And the best part is, anybody can contribute their own discoveries on the map (here's the guide). Very cool.
My only gripe is the map itself appears to be a scan of the poster they give out with the game. It would be awesome if Rockstar released a digital, pixel-perfect map with my favorite Google Map feature -- Street View.