What are the best plugins you use for WordPress and what you use them for ?

Akismet – best guard against comment spam for WordPress blog

BackUpWordPress - BackUpWordPress will back up your entire site including your database and all your files on a schedule that suits you.

All in One SEO Pack – Out-of-the-box SEO for your WordPress blog. Install the all in one seo pack or the “WordPress SEO by Yoast” plugin (you don’t need to install both)

WordPress SEO by Yoast – Yoast’s all in one SEO solution for your WordPress blog: SEO titles, meta descriptions, XML sitemaps, breadcrumbs & much more.

Google XML Sitemaps – This plugin generates a XML-Sitemap compliant sitemap of your WordPress blog. This format is supported by Ask.com, Google, YAHOO and MSN Search. The SEO plugins listed above can also do the sitemap for you.

Respected sir i'm a starter  here,  i just know few  plugins which i use .

hi, you will find full of best WordPress plugins. I think it will be very useful for your question: 

Must Use Plugins

• Akismet – best guard against comment spam for WordPress blog!

Database and File Backup Plugins

• BackUpWordPress - BackUpWordPress will back up your entire site including your database and all your files on a schedule that suits you.

Search Engine Optimization (SEO)

• All in One SEO Pack – Out-of-the-box SEO for your WordPress blog. Install the all in one seo pack or the “WordPress SEO by Yoast” plugin (you don’t need to install both).
• WordPress SEO by Yoast – Yoast’s all in one SEO solution for your WordPress blog: SEO titles, meta descriptions, XML sitemaps, breadcrumbs & much more.
• Google XML Sitemaps – This plugin generates a XML-Sitemap compliant sitemap of your WordPress blog. This format is supported by Ask.com, Google, YAHOO and MSN Search. The SEO plugins listed above can also do the sitemap for you.

Blog Security Plugin

• All In One WP Security & Firewall - The All In One WordPress Security and Firewall is the ultimate security plugin that will take your WordPress site’s security to a whole new level.
• BulletProof Security – BulletProof security plugin offers website security protection against: XSS, RFI, CRLF, CSRF, Base64, Code Injection and SQL Injection.

Blog Statistics Related

• Google Analytics for WordPress – This plugin makes it simple to add Google Analytics with extra search engines and automatic clickout and download tracking to your WordPress blog.
• WordPress.com Stats – Tracks views, post/page views, referrers, and clicks. This plugin requires a WordPress.com API key to function.

Caching Plugin

• W3 Total Cache - The fastest and most complete WordPress performance optimization plugin. After you use the w3 total cache plugin you won’t go back to using WP Super Cache.
• WP Super Cache - WP Super Cache is a static caching plugin for WordPress. It generates html files that are served directly by Apache without processing comparatively heavy PHP scripts. By using this plugin you will speed up your WordPress blog significantly.

RSS Subscription Related

• FeedBurner FeedSmith – This plugin detects all ways to access your original WordPress feeds and redirects them to your FeedBurner feed so you can track every possible subscriber.
• Add to Any: Subscribe Button – Helps readers subscribe to your blog using any feed reader.
• Category Specific RSS Menu – A simple WordPress plugin to add category specific RSS subscription menu into your posts, pages, sidebars. Very handy when your blog covers multiple topic from non related subjects. This plugin was developed by me.

Contact Forms Related

• Contact Form7 - Contact Form7 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.
• cforms – cforms II is the most customizable, flexible & powerful ajax supporting contact form plugin.
• WP Contact form – WP Contact Form is a drop in form for users to contact you.
• Gravity Forms - Build and publish your WordPress forms in just minutes. No drudgery, just quick and easy form-building.

Social Networking

• Sociable – Automatically add links to your favorite social bookmarking sites on your posts, pages and in your RSS feed.
• Share This – This is a very useful plugin that allows your visitors to share a post/page with others. Supports e-mail and posting to social bookmarking sites.
• Fat Free WordPress Social Share Buttons Plugin - This is a very simple and FAT FREE share button plugin that adds Google plus one, Facebook and Twitter share buttons to your blog posts.

For Google Adsense

• Google Adsense Plugin - Google AdSense Plugin allows you to set parameters of the ads displaying, such as format (text ad, image, text with an image or link), size, color of the elements in the ad block, rounded corners and the ad block position on the website.
• Simple Google Adsense insertion – Easy to use WordPress plugin to insert Google Adsense to your posts, pages and sidebar.
• Adsense-Deluxe – Place Google AdSense ads in your WordPress Posts. Capable of handling complex adsense management.

Photo Gallery Plugin

• Simple Photo Gallery - If you are looking for an effective, reliable, yet simple to use photo gallery plugin, then check this gallery plugin out. I use it whenever I need a photo gallery plugin.
• NextGen Gallery - The most popular WordPress gallery plugin and one of the most popular plugins of all time.

E-commerce, Paypal Donation and Payment

• WordPress Simple Paypal Shopping Cart – is a very easy to use plugin that allows you to sell products or services online in one click from your WordPress blog. Very useful when you want to sell eBooks or digital products from various posts and pages and need a WordPress Shopping Cart solution.
• WP Easy Paypal Payment Accept – This is a very easy to use WordPress plugin to Accept donation or Paypal payment for a service or a product in one click. Handy when you are trying to put a customized payment widget on your site (you can add it to the sidebar or a page or a post of your blog).
• WordPress eStore – This plugin provides a complete solution to sell products from your site. It is really good for selling digital products securely from your WordPress blog.
• WordPress Donations Plugin - The WordPress Donations Plugin allows you to collect donation for a cause from your WordPress powered site.
• PayPal for Digital Goods – This plugin allows you to use the PayPal for Digital Goods gateway to accept payment for digital goods. Provides inline checkout of digital goods.

Membership Plugin

• Simple Membership Plugin - The simple membership plugin lets you protect your posts and pages so only your members (free or paid) can view the protected content.
• WordPress eMember – WP eMember is a feature rich membership plugin that can turn your existing or new WordPress site into a powerful membership site.

Affiliate Related

• WP Affiliate Platform – WordPress plugin for affiliate recruitment, management and tracking. This plugin lets you run your own affiliate campaign/program and allows you to reward (pay commission) your affiliates for referred sales.
• WordPress Affiliate Link Manager - Allows you to automatically convert specific keywords from your blog posts and pages into your affiliate links. It also cloaks the affiliate links and gives it a prettier and shorter alias (good for affiliate marketers).

Other Very Useful Plugins

• Yet Another Related Posts Plugin -  This plugin gives you a list of posts and/or pages related to the current entry, introducing the reader to other relevant content on your site.
• Download Monitor - This plugin is very useful for managing and tracking your digital file downloads. You can track which file is being downloaded how many times and who is downloading them.
• Simple Tags – This is one of my favorite plugins. It makes my WordPress Tagging life so much easier. It helps with Auto completion, Suggested Tags, Tag Cloud Widgets, Related Posts, Mass edit tags.
• Executable PHP Widget – Like the Text widget, but it will take PHP code as well. Very handy for inserting custom ‘php’, ‘html’, ‘javascript’ in the sidebar.
• Crayon Syntax Highlighter – A plugin to highlight any code in your posts or pages. very handy if you use example codes in your Blog.
• Mailpoet Newsletters - Send newsletters, post notifications or autoresponders from WordPress easily, and beautifully.
• WP-Polls – Adds an AJAX poll system to your WordPress blog. Allows you to easily include a poll into your WordPress’s blog post/page.
• EWWW Image Optimizer - The EWWW Image Optimizer is a WordPress plugin that will automatically and losslessly optimize your images as you upload them to your blog.
• WP Video Lightbox - The WordPress Video Lightbox plugin allows you to embed videos on a page using lightbox overlay display. This plugin can be used to display images, flash, YouTube, Vimeo, iFrame etc in a nice lightbox overlay.

* From : tipsandtricks-hq

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many Thanks for all friends that answer that question

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The35 Best WordPress Plug-ins