The Leaper's Bridge is the shortest island crossing in Liberty City in Grand Theft Auto IV. It connects Algonquin and Colony Island. The name may come from the fact that the island it connects to, Colony Island, was in fact a leper colony. Other legends say that it's called Leaper's Bridge because people leap off the bridge to commit suicide, such as in one of the helitours the pilot will say it's "still the record holder for suicides," or something along the lines of that.
It is supposedly based off of the Roosevelt Island Bridge in New York City. However, it is not in the same location, and it bears no resemblance to the actual Roosevelt Island Bridge.