A light-weight module providing cross-vocabulary term relationships.
The Term Uncle module extends core taxonomy to allow you to attach "nephew" terms to an "uncle" term. It was created as a way to add Tags to Category terms, although it can be configured for many uses. Configuration is done the same way as content types, via additional checkboxes in the vocabulary edit page. If you want to enable Tags for the Article content type, edit the Tags vocabulary and select the Article checkbox; likewise select the Category checkbox under Vocabularies. Now edit a Category term and a Tags field will appear the same way it does when editing an Article node.