The RKD has various other websites. These can assist you in your research or for other purposes.
Rembrandt Database: this is a freely accessible English database with research material on paintings by and/or attributed to Rembrandt.
Art & Architecture Thesaurus (AAT): a structured list of terms which serves as a standard for setting up architecture, art and cultural historical collections
Iconclass: a multilingual subject classification system for the description and indexation of images of art works and other image material.
Central Register Design Archives (Centraal Register Vormgevingsarchieven, CRvA): This offers an overview of Dutch design archives. The database contains information on archives of designers, design companies, manufacturers, design galleries and designer associations.
Open Search Artists: an RKD web service for software developers. The system contains reliable biographical data on Dutch artists for use in for example collection management systems, apps or web applications.