A web site that has not been configured for SEO is not really a complete site.  This plugin will allow you to optimize each page for search engines.  This is very powerful if you know how to use it.

We use this plugin is 2 ways:

1. general setup for the homepage and site. The general settings located on the plugin main page which located under the “settings” left menu.

2. per page/post optimization can be found on each post/page bottom. Go to edit a page/post and you wil find the plugin in the bottom.

Description as written by the plugin author:

Some features:

• Support for Custom Post Types
• Advanced Canonical URLs
• Fine tune Page Navigational Links
• Built-in API so other plugins/themes can access and extend functionality
• ONLY plugin to provide SEO Integration for WP e-Commerce sites
• Nonce Security
• Support for CMS-style WordPress installations
• Automatically optimizes your titles for search engines
• Generates META tags automatically
• Avoids the typical duplicate content found on WordPress blogs
• For beginners, you don’t even have to look at the options, it works out-of-the-box. Just install.
• For advanced users, you can fine-tune everything
• You can override any title and set any META description and any META keywords you want.
• Backward-Compatibility with many other plugins, like Auto Meta, Ultimate Tag Warrior and others.

it can be downloaded from: http://wordpress.org/extend/plugins/all-in-one-seo-pack/