Flat folksonomy uses simple tags and has emerged as a powerful instrument for classifying and sharing a huge amount of knowledge on Web 2.0. However, it has semantic problems, such as ambiguous and misunderstood tags. To alleviate such problems, researchers have built structured folksonomies with a hierarchical structure or relationships among tags. Structured folksonomies, however, also have some fundamental problems, such as limited tagging of pre-defined vocabulary and time-consuming manual effort required to select tags. To resolve these problems, we suggested a new method of attaching a tag with its category, which we call a categorized tag (CT), to web content. CTs entered by users are automatically and immediately integrated into a collaboratively built structured folksonomy (CSF), reflecting the tag-and-category relationships supported by the majority of users. Then, we developed a CT-based knowledge organization system (CTKOS), which builds upon the CSF to classify organizational knowledge and enables us to locate appropriate knowledge. In addition, the results of the evaluation, which we conducted to compare our proposed system with the flat folksonomy system, indicate that users perceive CTKOS to be more useful than the flat folksonomy system in terms of knowledge sharing (i.e. the tagging mechanism) and retrieval (i.e. the searching mechanism).