All right, so “essential” might be a small exaggeration, but these are still plugins that every WordPress power-user should consider installing. And of course you’re a power-user, right?

1. After The Deadline

After The Deadline uses artificial intelligence to check spelling, style, and grammar in WordPress.

2. Akismet

Used by millions, Akismet is quite possibly the best way in the world to protect your blog from comment and trackback SPAM. It keeps your site protected from spam even while you sleep. I can vouch for its effectiveness, very little SPAM gets through. As of right now, Akismet has blocked 23,287 SPAM comments on my humble site.

3. All in One SEO Pack

The All in One SEO Pack Automatically optimizes your WordPress blog for Search Engines (Search Engine Optimization). Some features include support for Custom Post Types, automatic optimizing of titles for search engines and automatic META tags generation.

4. Contact Form 7

Contact Form 7 can manage multiple contact forms, plus you can customize the form and the mail contents flexibly with simple markup. The form supports Ajax-powered submitting, CAPTCHA, Akismet spam filtering and so on. I’m using this plugin on two pages; One-liners and About and it works as advertised.

5. FancyBox for WordPress

FancyBox for WordPress Seamlessly integrates FancyBox into your blog: Upload, activate, and you’re done. No further configuration needed. However, you can customize it from the Options Page if you like. It took quite a bit of tweaking for me to get this plugin to work exactly as I wanted, but as soon as I was happy with it, I started using it in every entry with pictures, for instance España, which I posted only a few days ago.

6. Google Analyticator

Google Analyticator adds the necessary JavaScript code to enable Google Analytics logging on any WordPress blog. This eliminates the need to edit your template code to begin logging. Google Analyticator also includes several widgets for displaying Analytics data in the admin and on your blog.

7. Google Authenticator

The Google Authenticator plugin for WordPress gives you two-factor authentication using the Google Authenticator app for Android/iPhone/Blackberry. If you are security aware, you may already have the Google Authenticator app installed on your smartphone, using it for two-factor authentication on your Gmail or Google Apps account. I covered the use of this plugin in June; How To Secure WordPress.

8. Google XML Sitemaps

The Google XML Sitemaps plugin will generate a special XML sitemap which will help search engines like Google, Bing, Yahoo and Ask.com to better index your blog. With such a sitemap, it’s much easier for the crawlers to see the complete structure of your site and retrieve it more efficiently. The plugin supports all kinds of WordPress generated pages as well as custom URLs. Additionally it notifies all major search engines every time you create a post about the new content.

9. Last.fm Recently Played Tracks

This is just for showing off my exceptional taste in music, and I’ve made some rather intrusive modifications to the original plugin. The result can be seen on the bottom of every page on this site. The original Last.fm Recently Played Tracks is still a great plugin if you want to display information from your Last.fm account on your WordPress site.

10. NextGEN Gallery

NextGEN Gallery is a full integrated Image Gallery plugin for WordPress with dozens of options and features. I don’t use galleries much when I post, so I tend to use WordPress’ internal image handling features and FancyBox for WordPress for visual effects, but if you’re handling lots of pictures and need to sort them into galleries, NextGEN Gallery should be right up your alley.

11. On This Day

On This Day is currently being used in the footer to display entires that has the same calendar date as today. A great way to keep your visitors clicking on your links.

12. Quotes Collection

The Quotes Collection plugin helps you collect, manage and display your favourite quotations in your WordPress site, like my One-liners collection.

13. Revision Control

WordPress comes with a revision feature, which is nice. But if you work with a post or a page for a long time you can get quite a lot of revisions and only a few of them are relevant. Tons of unnecessary revisions can make your database morbidly obese. Revision Control solves this problem by letting you configure how many revisions WordPress saves.

14. Shockingly Big IE6 Warning

Shockingly Big IE6 Warning is a plugin that shows a warning message alerting the user why it is bad to use IE6, the security risk and the bad compatibility of Web Standards. It might be time to disable this plugin soon, but about 5% of my visitors are still using IE6.

15. Similar Posts

Similar Posts displays a list of posts that are similar or related to the current posts. The list can be customized in many ways. Similarity is judged according to a post’s title, content, and tags and you can adjust the balance of factors to fit your own blog.

16. Subscribe To Comments

Subscribe To Comments is a robust plugin that enables commenters to sign up for e-mail notification of subsequent entries. The plugin includes a full-featured subscription manager that your commenters can use to unsubscribe to certain posts, block all notifications, or even change their notification e-mail address. You might also consider Subscribe to Comments Reloaded, which is updated more recently.

17. Twitter Tools

Twitter Tools is a plugin that creates a complete integration between your WordPress blog and your Twitter account. It can both send a Twitter message when you post a new entry and display your Twitter messages on your site. I use it to display my Twitter messages in the sidebar.

18. Wordbooker

The Wordbooker allows you to cross-post your blog posts to your Facebook Wall. You can also “cross polinate” comments between Facebook and your WordPress blog.

19. wp-days-ago

Shameless plug ahead (since this is a plugin I maintain myself): wp-days-ago displays the number of years, days, hours and minutes since a post or a page was published. Examples are “Just now” (less than a minute ago), “47 minutes ago” (less than an hour ago), “3 hours ago” (less than a day ago), “Yesterday”, “3 days ago”, “One week ago”, “76 days ago”, “2 years, 13 days ago” and so on. There are options for defining a prepending and appending text and change all the textual output from the plugin, for instance “minutes ago” and “One week ago”, making it easy for you to translate it to any language you want.

20. WP-Syntax

WP-Syntax provides clean syntax highlighting using GeSHi - supporting a wide range of popular languages. It supports highlighting with or without line numbers and maintains formatting while copying snippets of code from the browser. This is a great plugin if you post code examples on your site.

21. WP Super Cache

WP Super Cache generates static html files from your dynamic WordPress blog. After a html file is generated your webserver will serve that file instead of processing the comparatively heavier and more expensive WordPress PHP scripts. Another popular cache plugin for WordPress is W3 Total Cache.

22. WPtouch

WPtouch automatically transforms your WordPress blog into an iPhone application-style theme, complete with ajax loading articles and effects, when viewed from iPhone, iPod touch, Android, Palm Pre, Samsung touch and BlackBerry Storm/Torch mobile devices.