Google XML Sitemaps
This plugin will generate a special XML site-map which will help search engines like Google, Yahoo, Bing and Ask.com to better index your blog. Not only will this plugin auto generate your XML site-map but it will also automatically submit the updated site-map to Google, Yahoo, Bing, and Ask.com! Make sure your blog is found on major search engines by installing this plugin. I use it on all of my WordPress sites.
Google Analytics for WordPress
Google Analytics is the best way to know how many people are viewing your website. Google Analytics is not just a website counter, it gives you full analytics of what kind of traffic is on your site. Geographical location, time on site, referral sites, search engine info and more. After all if no one is viewing your blog what is the point in creating new post.
Wordbooker is a plug-in that will integrate WordPress to your Facebook page. You can have post automatically posted to your personal Facebook page or to your company Facebook page. You can also enable Facebook like buttons on all of your post automatically. Facebook like buttons are a great way to have viewers spreed the word about your blog by advertising your blog on their Facebook page.
WordPress Database Backup
Working in the IT field for over 10 years now I have seen users loose important information all to often. After all of the hard work you have put into your blog you don't want to loose all of it in the event of a server failure. I know most web server hosting companies automatically backup their servers but I definitely don't trust their backups or that they even do it at all. This plugin will automatically backup your WordPress database and email it to you on a scheduled basis. I wouldn't ever build a site without it!