The success of wikis for collaborative knowledge construction is triggering the development of a number of tools for collaborative conceptual modeling based on them. In this paper we present a reference architecture for wiki-based collaborative conceptual modeling tools. The characteristics of our reference architecture are: (i) the use of wiki pages to describe semantic terms and organisational mechanisms of a conceptual modeling language; (ii) the organization of wiki pages in an unstructured part and a structured part; and (iii) a multi-mode access to the pages. We also describe MoKi, a conceptual modeling wiki for ontologies and business processes fully compliant with the presented reference architecture.