Dougal Campbell

HeadMeta creates <code><meta></code> “description” or “keywords” tags for individual 
posts or pages. It can also create arbitrary <code><meta></code> or <code><link></code> tags. 
These are easily set on a per-post (or page) basis by simply adding Custom 
Fields.
See the Installation section for more detailed examples.

HeadMeta

Automatically add <link>, <meta> description and <meta> keywords to your HTML on a per-post (or page) basis.

gaxx

Automatically adds keywords and description meta tag on every page based on title, tags, categories, etc.
It works on the basis of defining some simple, tag-based rules for different page types (home, category, post etc).
Built on the foundations of the excellent Automated Keywords Generator by Julian Widya Perdana (http://mr.hokya.com/automated-keywords-generator)

Gaxx Keywords

Automatically adds keywords and description meta tag on every page based on rules that draw information from title, tags, categories etc

HeadMeta vs Gaxx Keywords