I'm constantly expanding the number and best of types of wordpress plugins used. Please leave some suggestions in the comments.
Here are a few of my favorites:
GoCodes: URL redirection/shortener plugin. Great for podcasting and redirecting affiliate program URLs.
Autoblogging Plugin for Wordpress Blogs:
WPRobot: This is a great tool for automatically integrating relevant Yahoo Questions/Answers, Amazon Product, Ebay products and more into your blog.
Fat Ping Wordpress Plugin:
Pubsubhubbub: This plugin automatically informs Google and sends them all of the content you publish. This sets your site as the original author, with anyone stealing your content receiving a duplicate content penalty.
Schema Wordpress Plugin:
SEO for Yoast: This plugin makes it easy to assign a schema to each part of your Wordpress blog or site.
All in One SEO Pack: This is a time honored plugin for organizing your sites SEO.
Google Site Verification
Google Site Verificaiton plugin makes it easy to verify your site with Webmaster tools.
Google XML Sitemap: This makes sitemap submission and updates easy. Also use Google XML Images as well.
Broken Links: checks your site for broken links and provides reports on what you need to fix.