The Williamsburg Bridge, the second bridge to connect Manhattan and Brooklyn, opened on December 19, 1903 to horse-drawn carriages, bicycles and pedestrians. However, due to complications between Greater New York and the privately owned railway companies, elevated trains did not run on the bridge until 1908. The final cost of the bridge and its approaches was $24.2 million, more than three times the original cost estimate.
The bridge not only served the traffic needs of a growing population, but also greatly affected migration patterns of ethnic groups. Before the bridge opened, first- and second-generation Irish and German settlers (who called the enclave "Kleine Deutschland") lived in the Williamsburg section of Brooklyn. When it opened, an influx of Jewish settlers from the overcrowded Lower East Side crossed the "Jews' Bridge" into Williamsburg. In turn, long-time residents moved out to Queens.
For a historic overview of the bridge: http://www.nycroads.com/crossings/williamsburg/
For a photo of the bridge that seems to date from 1903: http://www.americaslibrary.gov/jb/progress/jb_progress_wilbridge_2_e.html