Ontologies, formal specifications of domains, have evolved in recent years as a leading tool in representing and interpreting Web data. The OntoBuilder project supports the extraction of ontologies from Web search interfaces, ranging from simple search engine forms to multiple-pages, complex reservation systems. OntoBuilder enables fully-automatic ontology matching. The use of ontologies, as opposed to relational schema or XML, as an underlying data model allows a flexible representation of metadata, that can be tailored to many different types of applications. OntoBuilder was developed using Java, which makes it portable to various platforms and operating system environments. We demonstrate OntoBuilder using an easy-to-follow example of matching car rental ontologies. The system creates ontologies of car rental Web sites on-the-fly, and combine them into a global ontology. The benefits of OntoBuilder in resolving, in an automatic manner, semantic heterogeneity, including synonyms and designer errors are highlighted.