DBin is a Semantic Web application that enables groups of users with a common interest to cooperatively create semantically structured knowledge bases. These user groups, which we call “Semantic Web Communities”, are made possible by creating customized user environments called “Brainlets”. Brainlets provide user interfaces and domain specific tools (e.g. querying, viewing and editing facilities) which enable community participants to interact with the data of interest. Brainlets are directly created by domain experts using an XML description language. DBin clients communicate and exchange annotations using a P2P infrastructure. Access control and digital signatures put by DBin inside the authored RDF enable trust and information filtering. In this paper we show a specific use case where a “Semantic Web Community” is created to enable a group of users to share their del.icio.us tags and organize them into a cooperatively built RDFS ontology.