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.

svil4ok

Get tags on every single post (is_single()) and put them in meta keywords tag in header (wp_head()). Otherwise it’ll use the keywords define in $default_keywords. Get the_excerpt() and put it in meta description tag in header (wp_head()). Otherwise it’ll use the description define in $default_description.
Requires the wp_head() tag !

HeadMeta vs Meta Keywords & Description