Go4Expert - 20 Essential Wordpress Plugins

Wordpress is one of my favourite CMS but the power of Wordpress lies in its plug-ins and so let's share few of the best Wordpress plug-ins which I am using myself

All in One SEO Pack - Provides you with more options for your Wordpress blog for Search Engines. One of them is you can have a different title to your post that the actual title of your post. It helps in more than one way because you can make your post title for your users and page title for search engines.
SEO Smart Links - SEO Smart Links is not as popular plug-in as All in One SEO Pack but it is still one of the best SEO plug-in not only for search engines but for users as well. It automatically links to keywords and phrases in your posts / comments with corresponding posts, pages, categories and tags on your blog. It is perfect plug-in to get your blog interlinked.
Google XML Sitemaps - Google XML Sitemap will generate a special XML sitemap which will help search engines like Google, Bing, Yahoo and Ask.com to better index your blog and also it notifies all major search engines every time you create new content on your blog.
Yet Another Related Post Plug-in - When it comes to related posts plug-in there are quite a few of them but the one that I like the most is Yet Another Related Posts Plug-in because it has much better related matching algorithm and it also adds the related posts to your RSS feeds. The better the matching algorithm means an increase in page views per user on your blog.
Akismet - No matter how small or big blog you have you are bound to get comment spam and Akismet checks your comments against the Akismet web service to see if they look like spam or not and lets you review the spam it catches under blog admin.
Captcha Free - is one of my favourite plug-in because it actually blocks comment spam without opting for genuine users to go for CAPTCHA images and does not place any burden on the real commenter. This plug-in in the true sense stops spam and not users.
Google Analytics / Wordpress.com Stats - It is always good to have stats for your blog and what better than Google Analytics but to add to GA I also add Wordpress.com Stats which shows the posts and pages that are popular, the source of traffic, and what people click when they leave.
Subscribe To Comments - I am not sure why Wordpress does not have this featured built into Wordpress comments but thanks God we have a very robust plug-in that enables commenter's to subscribe for e-mail notification of subsequent comments.
Contact Form - This is yet another plug-in which I think should be part of the original CMS but nevertheless Contact form 7 is a brilliant plug-in which can help your blog to have contact form plus you can customize the form and content very easily.
WP-Super Cache - Generates static html files from your dynamic Wordpress blog and then uses the html file intelligently to show to your guests. If you are using shared hosting and your host is underpowered, or you're experiencing heavy traffic. Super Cached html files will be served more quickly than PHP but it should be used with care because if you change your coding often you may not be able to see the changes unless you clear the cache.
Login Lockdown - Enhances your site's security by recording the IP address and timestamp of every failed login attempt. If more than a certain number of attempts are detected within a short span of time from some IP, then the login function is disabled for all requests from that IP.
Secure Wordpress - This is yet another simple security enhancer plug-in which removes lot of unwanted data from guests like Wordpress version.
WP-Security Scan - Scans Wordpress blog for security vulnerabilities and suggests corrective actions for stuffs like passwords, file permissions ...
Advertising Manager - manages and rotates ads on your blog. The best part of this plug-in is you can select which ads to appear based on author, category, tag, page which means if you have few good niche categories where you have good related affiliate products you can opt for your own affiliate ads on those categories instead of Google Adsense.
Disqus Comment System - Apart from comments spam the other problem with Wordpress comments is it does not have a conversation based display for comments i.e. they are not organized into comments and their replies but disqus comment system can help you do exactly the same.
Lijit Search - When it comes to searching there is hardly anyone who can beat Google and lijit results are also powered by Google but what I like about Lijit is the way they show the results. You can test them out here at CodeItWell in the sidebar to see what I am talking about.
Outbrain - Outbrain is a blog posting rating system and is one of the simplest plug-in to have for rating your posts. Just in case you want to give it a test visit CodeItWell.
AddThis - There are so many social network plug-in's but with AddThis plug-in you can show which one you want them as default and make other go under the more link. See the working sample here at Go4Expert.com as well.
Ajaxed Wordpress - Improves user experience by implementing the power of AJAX to your wordpress powered blog.
WP-Automatic Backup - I normally do not prefer backup using front-end but still having a one that does all things automatically is a good choice. I do regular backups of my entire server but yet as there is an option of automatically do this I use it as well.

If you have more plug-in to share do so in comments below as that will make this list more extensive.