An Arch Bridge with two middle spans.
Ideal for when a big bridge would look silly, but a normal flat bridge looks unstable or its supports blocks the water way.

This structure is 3 block wide, 55 long, and 7 blocks high. Also laid with two lengths of minecart tracks, which are easily removed. File is in MCEdit schematic format, for ease of inserting into existing worlds.

Chop out one of the two middle spans to make a shorter bridge. Swap the stone brick and iron block for other materials.