1. All in One Seo PackThis plugin is far the most popular SEO plugin for WordPress and will boost you visibility in Search Engines, by giving you the ability to set-up custom titles, description and keywords for every page.
If you have WordPress 2.7.1 you might encounter some problems when you have set-up in Reading Options from Settings tab, the Front page to Home and the Posts page to Blog. You won’t be able to separate titles of those two pages just using conditionals between in template Header file and manually insert the titles for this Home and Blog pages. It is more a WordPress bug than a plugin bug.
2. Contact form 7I would say one of the most popular contact forms for WordPress for providing simplicity and versatility. You can fully customize and provides great included protection against spam, using Askimet plugin without necessary having to install Captcha, but you have that option also. You can even add the option to upload files.
3. Dagoon Sitemap GeneratorSome of its features include: support for multi-level categories and pages, category/page exclusion, multiple-page generation with navigation, permalink support, choose what to display, what order to list items in, show comment counts and/or post dates, and much more.
The plugin will give you a sitemap page in minutesm will keep the links inside tha page up to date and you won’t have to worry about it anymore.
4. Fancy BoxIf you feel that the popular Light Box plugin animations are too boring, you can try Fancy Box which is similar with Light Box but gives you more animation options.
After you install the plugin, it will apply Fancy Box to any image link, which opens an image. It uses jQuery so keep in mind that it might interfere with other plugin that is uses the same technology.
5. Google XML SitemapsThis plugin will create a Google sitemaps compliant XML-Sitemap of your WordPress pages and will inform all major search engines about any content update. This one also, is install and forget plugin and will help your SEO for sure.
6. Advanced ExcerptIn comparison with the similar plugin Evermore or default WordPress excerpt, which shorten the posts by paragraph, this one give you more control and let you chose a word or character count for a post preview.
7. WP e-CommerceThis shopping cart extension, I would say, is the most reliable free solution for WordPress.
If you want to take full advantage you will have to buy the paid version but totally worth it.
8. FlutterLater Edit – Now with WordPress 3.0 coming, I don’t see many reasons of using this plugin.
Also known as a very buggy CMS plugin, still remains a robust solution for those who want to use WordPress for content management system by creating custom write panels that can be fully customized (radio buttons, file uploads, image uploads, checkboxes, etc).
Flutter also includes power image manipulation (automatic resizing, cropping, watermarking, etc), edit in place (works instantly!), and modulation of your themes.