All In One SEO Pack – http://wordpress.org/plugins/all-in-one-seo-pack/
It is a brilliant plugin. It allows you to set the meta tags for each post, pages, and your site. If you would not like to index one of your pages, because it is private, you can set it up in 5 sec. It also have an input for your google webmaster, analytics and authorship information.
XML Sitemaps – http://wordpress.org/plugins/xml-sitemaps/
It is a great tool, because it generates sitemaps for your site and submits in to Google. Your site and pages get indexed faster.
Link Juice Keeper – http://wordpress.org/plugins/link-juice-keeper/
It is probably the most effective plugin, which is hard to replicate by simple coding. It collect the backlinks, which are going to pages of your site, which do not exist anymore and redirect them to your home page. With that you do not lose the link juice of your site, even after deleting pages, post on your site.