Content object tags at a content hosting service are used to classify stored content objects. Tags and clusters of tags (groups of one or more associated tags) can be recommended to a user of the content hosting service based on a user context, such as the browsing, viewing, uploading, or searching of content objects. Tags are scored based on content objects tagged with the tags in a targeted subset of content objects and a baseline subset of content objects, and based on the relevance of the content objects tagged with the tags. These tag scores can be weighted, and one or more tags can be selected for recommendation based on the weighted tag scores. Tag clusters can be selected for recommendation using a cluster hierarchy and determining whether a targeted subset of tags occur within a maximum number of tag clusters at a particular hierarchy level.