Mash-ups are an emerging paradigm for online applications that combine functionality from various sources. The Semantic Web, which extends the current Web with more support for data integration and data reuse, significantly eases the development of such mash-ups. But existing application frameworks offer only limited support for the integration of Semantic Web data. Additional needs for application development on the Semantic Web include bridging the semantic differences between the object-oriented and RDF(S) data models. The Semantic Web Application Framework, which extends the Ruby on Rails framework, enables rapid development of mash-ups. This article is part of a special issue on dynamically typed languages. © 2007 IEEE.