From the World Heritage inscription:
These are two of the finest contributions to Barcelona’s architecture by the Catalan art nouveau architect Lluís Domènech i Montaner. The Palau de la Música Catalana is an exuberant steel-framed structure full of light and space, and decorated by many of the leading designers of the day. The Hospital de Sant Pau is equally bold in its design and decoration, while at the same time perfectly adapted to the needs of the sick.
The building in the photo is of the Hospital de Sant Pau, which is only a few blocks away from the Sagrada Familia at the other end of Gaudi Avenue. If you are visting Barcelona, it is well worth your time to take the walk to visit the Hospital de Sant Pau. It was under reconstruction when I was there, but should be open to the public sometime in 2011.
View my complete list of UNESCO World Heritage Sites