The Centre de Données astronomiques de Strasbourg (CDS) develops a set of value-added services, widely used for information retrieval, observation preparation, data interpretation,... SIMBAD, VizieR, Aladin, and the ‘Dictionary of Nomenclature’, integrate heterogeneous, selected information from observatory archives, sky surveys and publications. Each service organizes information in a different way (astronomical objects, tables, images with overlays, nomenclature), and the CDS hub allows versatile information retrieval, e.g. looking for known information in a given region of the sky, including observations of ground- and space-based instruments, or searching by criteria in large data sets. Links among the CDS services, and with other reference on-line information systems, such as observatory and survey archives and publications, permit comprehensive searches in a wide variety of resources. Shared exchange standards and generic tools such as the GLU are essential for the building of links. XML is a tool for further information integration, and Aladin is a precursor of an integration tool, relying on FITS and XML. New functionalities will be developed at CDS in the context of the Virtual Observatory, e.g. for data mining and management of very large catalogues. A prototype set of interoperable archives will be implemented in the frame of the European Astronomical Virtual Observatory project.