The increasing number of Semantic Web applications that work with ontologies implies an increased need for building ontological knowledge bases. In order to improve ontologies during their development as well as to allow applications to be experimentally evaluated prior to their complete implementation and deployment, ontology bases must be filled with experimental data (i.e., instance ontologies), which can be used to evaluate methods used for information processing. We describe several approaches together with a method of building an ontological knowledge base for purposes of experimentation with Semantic Web applications. We also discuss characteristics and suitability of particular approaches to the development of experimental ontological knowledge bases.