The Centre de Donnees astronomiques de Strasbourg (CDS) develops a set of value-added services, widely used for information retrieval, observation preparation, data interpretation, etc. 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), and the CDS hub allows versatile information retrieval, e.g. looking for known information in a given region of the sky, including observations from 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 or 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 key 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.