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Orphan Fields module is a very tiny Drupal 7 auditing module that lets you know there is field data left behind when you uninstall certain modules.
For instance, if you install the Taxonomy module, then it will define a field type for tags, called field_tags. You can, of course, attach tags to just about any content type, including other tags. If you decide to uninstall the Taxonomy module later, however, this will leave all the tag data behind in the database.
The Orphan Fields module will show you which fields have been left behind in this way, and which content types are affected.
One way to see it in action is to do just what I mentioned above: Enable the Taxonomy module, create an article with tags, and then disable or uninstall the Taxonomy module. The Orphan Fields module will then show you a nag window at the top of every admin page, telling you that there are orphaned fields. The nag message will also provide a link to a report page, which gives some more detail and explains the problem.
Note that this module is a very hastily-written hack. It should never go into a production site.
This module was inspired by this discussion: http://drupal.org/node/943772