The 7.x version of the Taxonomy Autotagger module implements the interfaces created by the Tag API module, allowing a vocabulary to be searched against for suggestions of terms related to the content of a node.
Due to work constraints, the 5.x version of this module is no longer supported, whilst the 6.x version is only minimally supported.
An additional option is provided on a content type configuration page (e.g. admin/structure/types/manage/page). Under that option, it is possible to specify which text fields should be searched for terms in specified vocabularies, and which field should then be populated with the results. Note, it is possible to populate taxonomy term fields, and text fields.
As there were significant changes between Drupal 6.x and Drupal 7.x with respect to handling of tagging of nodes, there is no upgrade path for the Autotag module.