There exist many digital collections of cultural and historical resources, referred to as digital archives in this paper. Domains of digital archives are expanding from traditional cultural heritage objects to new areas such as popular culture and intangible entities. Though it is known that metadata models and authority records, such as subject vocabularies, are essential in building digital archives, they are not yet well established in these new domains. Another crucial issue is semantic linking among resources within a digital archive and across digital archives. Metadata aggregation is an essential aspect for the resource linking. This paper overviews three metadata-centric on-going research projects by the authors and discusses some lessons learned from them. The subject domains of these research projects are disaster records of the 2011 Great East Japan Earthquake, Japanese popular culture such as Manga, Anime and Games, and cultural heritage resources in South and Southeast Asia. The main goal of this paper is not to report on these projects as completed research, but to discuss issues of metadata models and aggregation which are important in organizing digital archives in the web-based information environment.