Dejan Batanjac

Creates meta description write panel for posts pages and custom post types. 
Meta description write panels are not present in the default WordPress installation.
Meta description were traditionally important for website SEO. Today, with the improvement of the software that reads and summarizes the textual documents, they are loosing the importance.
Still you can find them useful on pages with low or even no text.

SEO Meta Description

Creates meta description write panel for posts, pages and custom post types.

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

SEO Meta Description vs Gaxx Keywords