Built for the Expo '92 in Sevilla, the Alamillo bridge is an incredible beauty. It was concieved as one end of a mile-long pair of bridges connected by a viaduct. The other end of the viaduct was supposed to end with a sister bridge, giving the entire project symmetry. Unfortunately, the second bridge was never constructed. The extant bridge is fascinating all by itself however. The angle and weight of the tower requires no backstay to hold up the 500m long arch. Also see the interesting Calatrava bridge in Valencia, and the graceful Zubizuiri in Bilbao.

External link: unofficial Calatrava Site

Puente del Alamillo.
Sevilla, España.
Santiago Calatrava 1992.
Photo thanks to Aaron Sporer.