WordPress.org

This plugin automatically converts keywords in a post title to tags upon saving. It includes a user-editable list of words you want the plugin to ignore, which by default includes the more obviously-useless words like "I" or "wasn't." You can also reset the list back to defaults by deleting the current list.

• Converts keywords in post titles to tags
• Includes user-editable list of words to be ignored
• Ignore list can be reset to default at any time
• Converts on save, not publish.
• Does not convert if there are already tags assigned.

Multi-blog adminstrators take note: this plugin is especially helpful if you're building a community-based site where tagging is important and your bloggers are not always diligent about tagging.