jfoucher

This plugin uses the yahoo.com API to find the most relevant keywords from the content of your post, and then adds them as tags.
The tagging is fully automatic, so if you’re using a plugin like feedwordpress to display RSS feeds on your blog as posts, everything will get done as the feed posts are published. No user intervention necessary!

Auto-tags

Automatically add relevant tags to new posts.

M A Vinoth Kumar

buffercode

Through this plugin, user can able to combine the repeated tags under the single group tag name
<ul>
<li>Users can able to add repeated tags under a group name</li>
<li>Users can able to add up to Ten custom group name and tags</li>
<li>Users can select the group name in Add New Post section</li>
<li>After publishing the post, all tags under the respective Group name will be added to the tag section.</li>
<li>User can also edit the multiple tag in quick edit</li>
<li>User can also do bulk edit (new)</li>
</ul>
For More information : http://buffercode.com/multiple-tags-wordpress-plugin/

Multiple Tags

Through this plugin, user can able to combine the repeated tags under the single group tag name

Auto-tags vs Multiple Tags