This plugin automatically converts title and post contents to tags.
This plugin will convert every word that occurs in a post title or content into a term (tag). By default the plugin will add these terms to a new taxonomy called "words". Optionally, the terms can be added to the "post_tags" taxonomy, used for normal tagging in WordPress.
The plugin provides the option to create a so-called stop list. Words in this list will be kept (or even removed) from the taxonomy. This allows authors to ignore meaningless, but often occuring, words like 'a' or 'the'.
This plugin makes sure that the tags will actually cover a blog's content. This is great in combination with tag-clouds as the tag-clouds will better show what the blog is about.