George Stephanis

Simple 404 Keyword Insertion creates a page called “404-page” on activation. When a visitor hits a 404, the plugin serves that page with a <code>[404-keywords]</code> shortcode replaced by the sanitized keywords from the request URI. This helps create SEO-friendly 404 pages that reflect the content the visitor was looking for.

Simple 404 Keyword Insertion

Builds a custom 404 page based on the request URL, inserting the keywords from the failed request into your 404-page content.

techphernalia

This plugin helps your SEO by adding meta keywords tag to each page, post, archive (category, tag, date, day and year). 
Now it also allows you to specify common keywords to be included on every web page. Plugin from one of the best coder <a href="https://www.techphernalia.com/" rel="nofollow ugc">Durgesh Chaudhary</a>. 
For any support just leave your question at our <a href="https://www.techphernalia.com/meta-keywords-generator/" rel="nofollow ugc">Meta Keywords Generator</a> page.
Uses Title, Description, Categories, tags, Compulsary keywords (provided by you in setting) for keywords generation. Uses content after striping short codes and short tags for generation of description.
Reduces manual work of adding them manually.
Now more efficient by allowing you to add one time master keyword list.
A big Thanks to Omer Barlas for suggesting to change text box to textarea for better readability.

Meta Keywords Generator

Adds Meta keywords and Description to each and every page, post, archive.

Simple 404 Keyword Insertion vs Meta Keywords Generator