First of all, this post is not an introduction to the world of free WordPress plugins. Yes, they can be used to develop a seamless website/blog. But I also want to advertise the work of some excellent developers who need our donations to keep creating more innovative services!
Second, every day you can find any number of plugins that satisfy your needs. It is a significant decision choosing the right plugins. As any website designer and developer will tell you, it’s because they can effect the load time, theme, availability and efficiency of a website.
This list is my best as of “Today”. Here are some important plugins that not only load a website fast, but also make it more secure, search engine optimized, shareable and efficient:
Akismet checks your comments against the Akismet web service to see if they look like spam or not. It lets you review the spam it catches under your blog’s “Comments” admin screen. This plugin can help make sure you don’t create a website for spammers who need a place to advertise themselves.
Akismet or Automatic Kismet is a spam filtering service. It attempts to filter link spam from blog comments and spam Trackback pings. The filter works by combining information about spam captured on all participating blogs, and then using those spam rules to block future spam.
Why: Akismet uses the frequency power to catch spammers on your blog. And as more other websites use Akismet, the more the tool gets smarter at detecting which comments are spam.
Because I promised not to solely push WordPress plugins on you, I cannot outrightly approve WebsiteDefender for your website. But it does do the work of removing the wp-version, core update information, and blocking lock bad queries . With the help of Akismet it can decrease the number of spams you will get per month.
Why: I think many spammers use “robots” to generate their comments and these robots know how to generate a comment for a WordPress website. When you hide the version of your WordPress by using this plugin, these robots will have less power.
Search Engine Optimization
This is an incredibly valuable plugin that’s still, shockingly, free of charge. This plugin automatically creates meta-tags on each post page, allowing you to pre-establish any meta-titles or meta-tags you want included. All In One SEO is one of the best “plug & play” style plugins available – even if you’re new to WordPress and know next to nothing about SEO or WordPress development, all you need to do is install, activate, and write up a storm! There’s also a “pro” version available for $39 (for an unlimited license), which is allegedly the “#1 Most Downloaded WP Plugin.” They update frequently (sometimes too frequently), but it will definitely do its job of getting your website optimized for search engines. 
Why: This plugin is very user friendly in its set-up. Also, you can add a general pattern for all of your titles for posts/pages. It gives you the power to modify and customize (even turn off) the title, description and keywords per post/page.
Sometimes, links get broken, a page is deleted, a subdirectory forgotten, a site moved to a different domain. Most likely many of your blog posts contain links. It is almost inevitable that over time some of them will lead to a “404 Not Found” error page. Obviously, you don’t want your readers to be annoyed by clicking a link that leads nowhere. 
But even more importantly, broken links are big negative points for your website from a search engine’s point of view. For example, Google will categorize your website by broken links in “maintenance mode” and will decrease your ranking in its algorithm. This plugin will return a status of 200 (Successful) for pages that don’t exist, which tells Googlebot that the requested URLs are valid and should be indexed. 
Why: Look at this post, and how many reference/links I have used in it. This plugin (frequently) checks all of these links. If a reference link is redirected, it will change my post link to the redirected link. If the reference link does not exist, it will simply remove the hyperlink so as to not confuse visitors or search engines.
This plugin generates a XML-Sitemap (compliant sitemap) of your WordPress blog. This format is supported by Ask.com, Google, YAHOO and MSN Search.
Why: In simple words, this plugin helps search engines read your website’s structure and therefore index it better.
You need a related post plugin to lead visitors to other content. As well, you need to create more internal links that are so sought by search engines. I prefer to have related posts on sidebars but this plugin will create hyperlinks at the end of your posts/pages.
Why: I have found this plugin “efficient” to find the right related posts.
Google Analytics for WordPress (optional)
This plugin simply adds your Google Analytics Code to your website if you are interested in monitoring your visitors’ behavior.
Why: With Google Analytics for WordPress, you can easily add your Google Analytics Profile to your website by permitting granted access to your profile.
Call to action (CTA)
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 and Akismet spam filtering.  This plugin is always available on the popular plugin’s page.
Why: This plugin provides the service of generating a new contact form that stays user friendly and simple. The only issue I have is that it uses a hosting server to send email and there is no synchronization with 3rd parties.
DISQUS is a comments platform that helps you build an active community from your website’s audience. It has awesome features, powerful tools, and it’s easy to install.  It even has a cool theme that complements your mobile theme.
Why: By using this plugin, you can enjoy the power of DISQUS.com to manage your comments or translate the theme.
MailChimp helps you design email newsletters, share them on social networks, integrate with services you already use, and track your results. It’s like your own personal publishing platform.  If you are using other email marketing platforms, you should find a way to add it directly to your WordPress website. MailChimp can be connected with this plugin.
Why: For me, this plugin is better than embedded codes of the MailChimp because it does not redirect visitors to other pages.
Performance and Sharing
Cache plugins take a real-time snapshot (building a cache) of every Page, Post, Category, Link, etc. These snapshots are then stored (cached) intuitively, so they can be referenced later, in order to save all of that processing time that has been dragging your site down and costing you money.
Why: I have tested many websites with different cache plugins (or without) and tested the loading time with Pingdom. Obviously a cached website tends to outperform its non-cached counterparts. Quick Cache’s setting page is very user friendly in contrast to the W3 Total Cache which has a higher popular rating among WordPress users. But try both and see which one works best for you.
Surfing habits have changed in recent years. People are more likely to access the net on their smartphones or their tablets. You should be ready to service them by having a better design for your website.
WPtouch automatically transforms your WordPress website into an application-like theme. It offers a complete loading of articles and effects when viewed from the most popular mobile web browsing devices like the iPhone, iPod touch, Android mobile devices, Palm Pre/Pixi and BlackBerry OS6 mobile devices. 
Why: Do you know any other option? I find it works well; but not when you are using a cache plugin.
What other plugins do you offer?
As always, I appreciate your opinion and your comments about my offered plugins. But if you have any others of note that I should know about, feel free to mention them here. It’s a small World Wide Web, after all.
This post is edited on Jan 16, many thanks to Warren H for his comment.