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.

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.

HeadMeta vs Quick META Keywords