We present SOBOLEO, a system for the webbased collaborative engineering of SKOS ontologies and annotation of web resources. SOBOLEO enables the simple creation, extension and maintenance of taxonomies. At the same time, it supports the annotation of web resources with concepts from this taxonomy. Categories and Subject Descriptors H.5.3 [Information Systems]: Group and Organization Interfaces; H.3.3 [Information Systems]: Information Search and Retrieval; H.3.5 [Information Systems]: Online Information Services; H.4.3 [Information System Applications]: Communications Applications General Terms Design, Human Factors Keywords Ontology engineering, social software, collaboration, annotation, SOBOLEO