John Sear

Adds the Google-specific metatag “news_keywords” with Post Tags into wp_head() to improve your chances of appearing in Google search results for relevant queries. See also http://support.google.com/news/publisher/bin/answer.py?answer=68297 🙂

News Keywords

Still Works

Adds easily the Google-specific metatag "news_keywords" with Post Tags into wp_head().

Timo Schlueter

TagMeta generates meta keywords using your most used tags. 
It automatically gets the 10 most used tags out of your 
blog posts and pages and puts them into a valid meta- 
keyword string. This string then appears in the header of 
the home page of your theme. If you want to add tags per 
post, i suggest youre going to use the Headspace plugin.

TagMeta

TagMeta generates meta keywords using your most used tags.

News Keywords vs TagMeta