WordPress is just a regular old CMS/blogging platform without plugins - feature-based add-ons for extending functionality. Plugins are undoubtedly the only way to really customize the experience for users and improve the visibility of a blog's Web presence. Today let's look at some of the best plugins - from A to Z. While we could not manage to find too many plugins for X, Y, Z (and a few other letters), we hope you get the general idea. If you have your creativity cap on and can think of some categories for the letters we missed, or if we happened to miss one of your favorite WP plugins, just comment below and we'll update this post (and reference you and your site with a link).

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Best Wordpress Plugins for Audio
Podcasting and the use of audio may not have the draw that it once did but it remains prevalent and is in fact used heavily in certain industries. Providing the best quality audio is only half the battle; you will also need a seamless, pleasing way to present it to users. Let these plugins support that objective.
Recommended Plugins: Audio Player, MediaElement.js - HTML5 Video & Audio Player, Audio Player Widget, Audio Link Player

Best Wordpress Plugins for Backups
When it comes to backups, it's always better to be safe than sorry. The sad truth is that the vast majority of bloggers simply don't back up, which as they quickly learn, can cause more headaches than necessary. Take a few minutes each week to backup your WordPress site or use one of these plugins to automate the task.
Recommended Plugins: EZPZ One Click Backup, WordPress EZ Backup, XCloner - Backup and Restore

Best Wordpress Plugins for Comments
One of the clearest signs of a successful blog is the number and veracity of comments from visitors. It's not easy to elicit comments but they do provide other users a sense of the value in the community. If you are considering ramping up your comment acquisition efforts, consider these WP plugins for comments.
Recommended Plugins: IntenseDebate Comments, Disqus Comment System, Facebook Comments for WordPress, Comment Rating

Best WordPress Plugins for Donations
WP is used in nearly every industry. Thanks to it's open nature, it is even quite common in the non-profit realm. Since these entities exist primarily on donations, if you are interested in requesting and processing funds from visitors, let these plugins support that objective.
Recommended Plugins: PayPal Donations, Donation Can, Donate Plus, Mingle Donations Button

Best WordPress Plugins for Email Subscriptions
Blogs and email go together like peas and carrots. Since it is unreasonable to expect that every one of your visitors will subscribe to an RSS feed or even visit your site everyday, there is no greater compliment to a weblog that a well devised email marketing strategy. Let these plugins help get you started.
Recommended Plugins: MailChimp List Subscribe Form, Constant Contact for WordPress, AWeber Web Form Plugin, WP Email Capture, Subscribe2

Best Wordpress Plugins for Forms
Every website needs a contact form and blogs are no different. Having forms for general and specific requests that can be quickly and easily created are the sign of a well-developed website. If you don't have a contact form (or two or three), stop what you're doing and get started with these WP plugins.
Recommended Plugins: Contact Form 7, Fast Secure Contact Form, Custom Contact Forms, Formiddable Forms

Best Wordpress Plugins for Images/Galleries
Photo galleries are excellent ways to increase time on site and build loyalty. When blog owners use images as assets (and more broadly, galleries as showcases of these image assets) it's easier to increase exposure for brands and increase brand interaction. Let the following plugins support that mission.
Recommended Plugins: NextGen Gallery, Page Flip Image Gallery, WP Photo Album Plus, Flickr Photo Album, SEO Friendly Images

Best Wordpress Plugins for Links
The Web is not a "web" at all without links. Maintaining the many links that bloggers create on a daily, weekly or monthly basis however can become cumbersome - even for the most organized.  Automating the maintenance proves useful in relation to customer experience and overall SEO efforts.
Recommended Plugins: SEO Smart Link, Broken Link Checker, WP Render Blogroll Links

Best WordPress Plugins for Managing Ads
Bloggers can't make money without selling something - and that includes advertising space. Managing advertising creative and placements is often best left to technology. Let the following plugins support your quest to towards blog profit with these WP plugins for managing ads.
Recommended Plugins: Simple Ads Manager, WP125, AdRotate, Ozh’ Who Sees Ads

Best Wordpress Plugins for Optimization
Everybody wants a well-optimized Web property and why not - the benefit of natural traffic is well documented.
Recommended Plugins: Platinum SEO Pack, Greg’s High Performance SEO, WordPress SEO by Yoast, SEO Ultimate

Best WordPress Plugins for Polls
If there's one tactic that works to get peoples attention and drive traffic to a Web property/weblog, it is the use of polls/surveys. While best suited to those with a large, active community, when well promoted polls and surveys are an interactive way to get site visitors involved.
Recommended Plugins: SodaHead Polls, PollDaddy Polls and Ratings, WP Polls

Best WordPress Plugins for Ratings
Search engines are placing greater focus on the presence and quality of ratings and reviews. While the introduction of schema.org and the increased demand for structured data present blog operators with a measurable opportunity, you still need comments, ratings and reviews to work with. Let these WP plugins help you begin the process.  
Recommended Plugins: Comment Rating, GD Star Rating, Star Rating for Reviews

Best WordPress Plugins for Social Media
Social media is here to stay (what form it will take in the future is another matter). Whether Facebook, Twitter, or one of the myriad smaller niche social media destinations, offering users the ability to help you spread/distribute message is standard. Not doing anything in the social realm puts you in the minority so use the plugins to help users start socializing.
Recommended Plugins: Sexy Bookmarks, Simple Facebook Connect, WP Tweet Button

Best Wordpress Plugins for Translation
The opportunities to gain new customers (and increase sales) is only possible through internationalization. While few sites delve deeper into translation than through automated means, something is better than nothing (despite those that 
Recommended Plugins: Global Translator, Google Ajax Translation, Transposh, qTranslate

Best Wordpress Plugins for Users (Membership Community)
Successful Web enterprises have one thing in common - loyal, active communities. Start a blog with membership/community features available in the first iteration and you will be positioned well to keep growing your site, and brand. Consider the following WP plugins for your own membership-based Web property.
Recommended Plugins: s2Member, Mingle, WP-Members

Best Wordpress Plugins for Video
Video is not the next big thing - it is the channel that marketers and consumers are most excited about right now. Video can be challenging to implement and even more cumbersome to manage so use the following plugins to get started the right way.
Recommended Plugins: VideoPress, All-in-One Video Pack, WordPress Video Plugin, Stream Video Player, MediaElement.js, Embedded Video

Best Wordpress Plugins for WordPress Health
Thanks to the sheer number of WP deployments, it has become a very visible target for spammers. The health of your Wordpress installation is vitally important, so important in fact that Google has even begun sending warnings to those WP sites that may have security vulnerabilities. Stay healthy with these two essential plugins.
Recommended Plugins: WP Security Scan, Akismet