Extending Web services with semantic contents or data is an error-prone, time-consuming and costly task. It requires a lot of effort from service suppliers in terms of expertise and training in cognitive or knowledge engineering. The presence of multiple semantic annotation formalisms for Web services make this task even more confusing and complicated. In some cases, the supplier-provided semantics are not completely related to the factual usage of services. Users may perceive differently the capabilities of a service from the ones recommended by suppliers. In this paper, we present a semantic tagging method for Web services that is easy to realize, low-cost and based on user participation and usage experience. Service users can annotate any Web service that she/he tested or used with her/his own vocabularies to reflect the functional capability of the corresponding services. We propose a collaborative tagging system to support the annotation activities using our model.