These are the plug ins I use on both client sites and my own. They provide the majority of the system requirements to make sure your website is optimised fully for the search engines and additional functions to make life easier.

Back Up

http://lesterchan.net/portfolio/programming/php/#wordpress-wp-dbmanager

Site Speed

http://wordpress.org/extend/plugins/w3-total-cache/

External linking

http://www.semiologic.com/software/external-links/Related posts

Related posts

http://wordpress.org/extend/plugins/yet-another-related-posts-plugin/

Keyword phrase internal linking

http://www.prelovac.com/vladimir/wordpress-plugins/seo-smart-links

On Page SEO

http://wordpress.org/extend/plugins/all-in-one-seo-pack/

Author mark up

http://www.authorsure.com/

Content curation

http://www.target-info.com/mycurator/

Adding keywords to images

http://www.prelovac.com/vladimir/wordpress-plugins/seo-friendly-images

Broken link checker

http://wordpress.org/extend/plugins/broken-link-checker/