During the last years, the cloud computing technology has emerged as a new way to obtain computing resources on demand in a very dynamic fashion and only paying for what you consume. Nowadays, there are several hosting providers which follow this approach, offering resources with different capabilities, prices and SLAs. Therefore, depending on the users' preferences and the application requirements, a resource provider can fit better with them than another one. In this paper, we present an architecture for federating clouds, aggregating resources from different providers, deciding which resources and providers are the best for the users' interests, and coordinating the application deployment in the selected resources giving to the user the impression that a single cloud is used. ; This work is supported by the Ministry of Science and Technology of Spain and the European Union under contract TIN2007-60625 (FEDER funds), the Ministry of Industry of Spain under contract TSI-020301.1009.3 (Avanza NUBA project) and Generalitat de Catalunya under contract 2009-SGR-980 ; Peer Reviewed