ivan-ovsyannikov

The plugin automatically adds keywords in heading from tags. Uses for this purpose one SQL-query as <code>get_the_tags()</code> works only in Loop. The ‘Quick META Keywords’ plug-in uses categories for keywords, this plug-in uses tags. The ‘Tags2MetaKeywords’ plug-in does not add automatically.

Tags2Keywords

The plugin automatically adds keywords in heading from tags.

P. Chandra

Automatically adds a META keywords tags with keywords within the html HEAD tags. The categories are used as keywords. 
Note: Works on single posts only as they are assigned categories.

Quick META Keywords

Automatically adds a META keywords tags with keywords within the html HEAD tags. The categories are used as keywords.

Tags2Keywords vs Quick META Keywords