Search Engine Optimization & Metrics
If you have a website, you want to make sure people are finding it. While effective SEO is more complicated than simply installing a couple of plugins, a good first step in the process is ensuring Google and other search engines index your site properly.
This plugin is pretty much a universal recommendation from everyone who uses WordPress. It offers a lot of settings that will help you milk more SEO out of your site. Using the basic settings, such as setting keywords, is simple; some of the other settings will take a bit more research and tweaking.
There are two kinds of sitemaps. One is a human-readable index of your site that offers an alternative for navigation. Unlike that kind of sitemap, the Google XML sitemap plugin generates an XML sitemap, invisible to your visitors, but which will help search engines like Google, Yahoo, Bing and Ask.com index your blog.
Akismet is automatically included with WordPress and is maintained by Automattic, the company that is responsible for WordPress itself. If you enable comments on your site, it’s an invaluable addition, as it runs a spam filter to check comments against the Akismet web service. You need an API key to use it, but this is free for many users and easy to set up.
Every site needs regular backups; a blog, or other sites with regularly-changing content, doubly so. This plugin sends you automatic backups to your email (I set mine for weekly). If your site goes down or gets hacked, you’ll be glad you have a backup ready to deploy.
WordPress automatically creates a feed for your blog posts, but it doesn’t really let you do much with it, and in my experience that feed isn’t always reliable on other services like Facebook. At any rate, I’ve had better luck with Feedburner. This plugin allows you to redirect your feed to your feedburner account.
This plugin allows you to add your choice of share buttons to the bottom of your posts/pages, including all the major social media sites as well as email and print buttons. It also gives you buttons for people who might want to follow your Facebook, LinkedIn, Twitter or other accounts.
Another, and very popular, plugin that allows you to add links as well as an RSS feed to your favorite social bookmarking sites.
There are a number of contact form plugins out there. I like this one because, while a little clunky, it does the job. You can decide on which fields you need, whether you want check boxes or other input fields, and can tailor the messages. Once you set it all up, the form will send an email to the address you designate. You can also have multiple forms on one site. It creates a shortcode. a block of text in  brackets, that you insert wherever you want the form to appear.
How to find plugins
While I’ve linked to the plugin sites, all of these should be available through searching for them by name in the plugin directory within the WordPress dashboard. They should all be compatible through at least WordPress 3.0.1.
There are hundreds of plugins for WordPress, so clearly this is just the tip of the iceberg. Finding the right one can be difficult, so word of mouth can help narrow down the options. With that in mind, if you know of other indispensible plugins, please let me know.