WordPress plugins are incredibly powerful and can make your website or blog fully featured and dynamic.
There are some plugins that I always install on all my WordPress sites because they are so useful.
This plugin (a fork of StatPress Visitors) shows the real-time statistics on your blog. It collects information about visitors, spiders, search keywords, feeds, browsers, OS, etc., as Statpress Visitors. – additifstabac.free.fr
Counterize is a complete counter and statistics plugin for WordPress that does not use external libraries. – http://www.gabsoftware.com
This plugin lets you show a different greeting message to your new visitors depending on their referrer url. For example, when a Digg user clicks through from Digg, they will see a message reminding them to digg your post if they like it. – http://omninoggin.com
WordPress SEO is the most complete WordPress SEO plugin that exists today for WordPress.org users. It incorporates everything from a snippet preview and page analysis functionality that helps you optimize your pages content, images titles, meta descriptions and more to XML sitemaps, and loads of optimization options in between. – http://yoast.com
The Better SEO Slugs makes your post and pages permalinks more SEO friendly. It removes common words that are unnecessary for search engine optimization from your post URL. It doesn’t shorten your slug in a random manner. It only deletes the words which are not important in terms of SEO. – http://twoenough.com/
Google’s Font Directory is one of the most valuable contributions to web typography in a really long time. By hosting an assortment of free open source fonts, they have made it practical for anyone to add rich typography to their website with very little hassle. – http://adrian3.com
We use artificial intelligence and natural language processing technology to find your writing errors and offer smart suggestions. – http://www.afterthedeadline.com/
This concludes my list of WordPress Plugins that I always install. If you know of any other great WordPress plugins I am missing out on, let me know in the comments!