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.

Matthew Poer

We’ve created a WordPress Plugin which adds site-wide Keyword and Description meta-tags. We’re using it on this web site!
The process is simple: define some site-wide description and keyword meta-tags. These will be used everywhere unless you (or your authors) choose to append keywords or override them completely for a particular post or page.

PSI Meta Tags

Add Meta-Tags to your site, and customize them on a post-by-post basis

TagMeta vs PSI Meta Tags