We have worked with hundreds of wordpress plugins. These were the best WordPress plugins (free) carefully selected simply because they proved to be the most flexible, usable, stable and useful. Some of them are essential installations – One thing to remember, Worpdress runs smoother with fewer plugins. so install ONLY what you need!

Social and Commenting

Digg Digg

With Digg Digg by Buffer, you have an all in one social sharing plugin for your blog. Display all social sharing buttons nicely on your blog and make it look amazing, just like Mashable.
Display all popular social sharing buttons with count, such as Twitter, Buffer, Facebook Share, Facebook Like, Digg, LinkedIn, Google +1, Reddit and more.

Facebook Like Box

Facebook Like Box enables you to display the facebook page likes in your website.

Livefyre Comments 3

Livefyre Comments 3 is a comment platform for real-time conversations. Comments 3 centralizes conversations from around the social web back to your site, and
encouragwa engagement between readers on your site to build community around your content.
Livefyre’s Admin dashboard allows you to track and monitor every interaction on your site from one location, in real-time.

ClickDesk Live Support – Live Chat – Help Desk Plugin for Websites

Besides being the fastest live chat service, ClickDesk also allows web visitors to click to call and voice chat with you directly from browser or by dialing our local access numbers available in 40+ countries. Manage your emails & offline chats with the new integrated help desk. Agents can now answer live chats and calls from online agent panel, Google talk or Skype.

eCommerce

Tribulant Checkout – Shopping Cart Plugin

Sell digital- and tangible products on your WordPress website with multiple payment methods, shipping calculation and data capture. This is our top choice for eCommerce. Not as popular as wooCommmerce but definitely better. Some features we like are bundle creation of product variation before attaching categories. It also have file upload field and a great custom field input attached with price for engraving.

WooCommerce – excelling eCommerce

Transform your WordPress website into a thoroughbred eCommerce store with WooCommerce eCommerce plugin. Delivering enterprise-level quality and features, WooCommerce also offers premium eCommerce themes and extensions to further enhance your shopfront.

Shopp

Shopp adds an infinitely flexible WordPress ecommerce plugin and secure shopping cart that runs thousands of successful online storefronts. Shopp is developed by a community of volunteers to provide a solid e-commerce toolkit with the best engineered framework for developers to build upon.

WP eCommerce

WP e-Commerce remains the most popular and a robust e-commerce plugin for WordPress and is the ideal way to sell products, downloads or services online. Whether you are setting up a WordPress store for yourself or are a WordPress developer setting up a store for a client, WP e-Commerce offers the most features and capabilities of any store plugin. WP e-Commerce comes standard with many features such as multi-tier pricing for quantity discounts.

Site Development Tools

WP-phpMyAdmin

Provides phpMyAdmin from the WordPress admin console

WP-Options-Manager

With this plugin you can browse, delete and debug the entrys of your WordPress options database table. Important: Only the administrator user should have access to this plugin and should know what he is doing!

Broken Link Checker

Checks your blog for broken links and missing images and notifies you on the dashboard if any are found.

Maintenance Mode

Adds a splash page to your blog that lets visitors know your blog is down for maintenance. Logged in administrators get full access to the blog including the front-end.

Photography & Galleries

NextGEN Gallery is the most popular WordPress gallery plugin, and one of the most popular WordPress plugins of all time, with over 9 million downloads.
It provides a powerful engine for uploading and managing galleries of images, with the ability to batch upload, import meta data, add/delete/rearrange/sort images, edit thumbnails, group galleries into albums, and more.

Smush.it

Yahoo’s excellent Exceptional Performance series recommends optimizing images in several lossless ways: stripping meta data from JPEGs, optimizing JPEG compression, converting certain GIFs to indexed PNGs, stripping the un-used colours from indexed images, Smush.it offers an API that performs these optimizations.

Search Engine Optimization

WordPress SEO by Yoast

The first true all-in-one SEO solution for WordPress, including on-page content analysis, XML sitemaps and much more.

All in One SEO Pack

Out-of-the-box SEO to optimize you WordPress blog for SEO (Search Engine Optimization). Options configuration panel, XML Sitemap support – submit your sitemap to Google and Bing and improve your SEO, Google Analytics support, Support for SEO on Custom Post Types, Advanced Canonical URLs, Fine tune Page Navigational Links, Built-in API so other plugins/themes can access and extend functionality.

Google XML Sitemaps

This plugin will generate a special XML sitemap which will help search engines like Google, Yahoo, Bing and Ask.com to better index your blog.

404 Redirected

A smart 404 redirection plugin that provides the ability automatically or manualy redirect visitors to most relevant content, and provides page suggestions when relevant content can not be found. This is useful to prevent ranking and traffic drop on bookmarked pages when url/permalinks are modified.

Google Analytics Dashboard for WP

Displays Google Analytics Reports and Real-Time Statistics in your Dashboard. Automatically inserts the tracking code in every page of your website.

Optimize

W3 Total Cache

The only WordPress Performance Optimization (WPO) framework; designed to improve user experience and page speed. Recommended by web hosts like: Page.ly, Synthesis, DreamHost, MediaTemple, Go Daddy, Host Gator and countless more.

WP Super Cache

This plugin 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.
The static html files will be served to the vast majority of your users, but because a user’s details are displayed in the comment form after they leave a comment thos

Backup

UpdraftPlus – Backup/Restore

take backups locally, or backup to Amazon S3, Dropbox, Google Drive, Rackspace, (S)FTP, WebDAV & email, on automatic schedules. Best of all you wont need to use time consuming ftp to upload files or log into MySQL admin to restore the database. Useful if you mess up your site and site is infected.. Restore your website with a click..

Limit Login Attempts

Limit rate of login attempts, including by way of cookies, for each IP.

Security and Malware

Wordfence Security

Anti-virus, Firewall and real-time WordPress security Network. This is the most recommended plugin for WordPress Security.

Sucuri Security – SiteCheck Malware Scanner

Malware Scanner plugin enables you to scan your WordPress site using Sucuri SiteCheck right in your WordPress dashboard. SiteCheck will check for malware, spam, blacklisting and other security issues like .htaccess redirects, hidden eval code, etc.

WP Easy Paypal Payment Accept

Easy to use WordPress plugin to accept paypal payment for a service or product or donation in one click. Can be used in the sidebar, posts and pages.

Core Functionality

Enable Media Replace

Enable replacing images, media files by uploading a new file in the “Edit Media” section of the WordPress Media Library. This is useful when for example you change your logo, and want it automatically replace all occurrence in the website.

Gravity Forms

Easily create complex and conditional web forms and manage form entries within the WordPress admin.

The Events Calendar

Create and manage events with ease. Get professional-level quality and features backed by a team you can trust. The Events Calendar will help you kick ass.
The Events Calendar is built and supported by WordPress artisans Modern Tribe. Looking for additional functionality including recurring events, ticket sales, publicly submitted events, new views, Facebook event integration and more?

Members

Members is a plugin that extends your control over your blog. It’s a user, role, and content management plugin that was created to make WordPress a more powerful CMS.
The foundation of the plugin is its extensive role and capability management system.

SpeakPipe – Voicemail for Websites

SpeakPipe allows your customers, podcast listeners, blog readers and fans to send you voice messages (voicemail) right from a browser without any phone calls.

Print Friendly and PDF Button

The Print Friendly & PDF button saves paper and ink when printing or creating a PDF. It’s fast, easy, and looks great when printed. Add the button now, and your users will see the difference. The Print Friendly & PDF button automatically creates printer friendly and PDF versions of your pages without the hassle of having to create a print CSS file.

For more Plugins, check Make use of “The Best WordPress Plugins”
http://www.makeuseof.com/pages/best-wordpress-plugins