These are the current (March 2013) best plugins that I use with WordPress websites:

Calendar Plugins:

All-in-One Event Calendar by Timely (Event Calendar with Widget) -http://time.ly/ - (get their Premium Version for Free – only offered through their website) show a calendar in your sidebar with upcoming events, a calendar page with poster board dates

Image/Media Plugins:

Dynamic Content Gallery - http://www.studiograsshopper.ch/dynamic-content-gallery/ - show a slideshow of images from your current WordPress posts or use with All In One Event Calendar for images of Calendar or Music Events

Slim Jetpack - http://wordpress.org/extend/plugins/slimjetpack/- turn your native WordPress gallery into a beautiful slideshow.  All the good features of Jetpack without the hassle of a WordPress account.

Easy FancyBox - http://wordpress.org/extend/plugins/easy-fancybox/ - easily enable the FancyBox jQuery extension on all image, SWF, PDF, YouTube, Dailymotion and Vimeo links. Also supports iFrame and inline content

Electric Studio Auto Post Expire - http://www.electricstudio.co.uk/ - for use with All In One Event Calendar by Timely – automatically expires posts by setting the post to “Draft” instead of “Published” after a certain date

Advanced YouTube Embed Plugin – Embed Plus - http://wordpress.org/extend/plugins/embedplus-for-wordpress/ – Best WordPress plugin for YouTube Videos

MediaElement.js – HTML5 Audio and Video - http://wordpress.org/extend/plugins/media-element-html5-video-and-audio-player/ - play videos uploaded to your website

Portfolio Slideshow - http://wordpress.org/extend/plugins/portfolio-slideshow/ - easily rotate images with smooth transitions and thumbnails

Resize At Upload Plus - http://wordpress.org/extend/plugins/resize-at-upload-plus/- resizes images as you upload them through WordPress – great for use with Dynamic Content Gallery

Other “Must Have” Plugins:

Leaflet Maps Marker - http://wordpress.org/extend/plugins/leaflet-maps-marker/ - custom markers and layout for Google Maps, Bing Maps, Social Sharing

Contact Form 7 – http://wordpress.org/extend/plugins/contact-form-7/ – a great way to customize a contact page or create surveys, applications and forms

W3 Total Cache - http://www.w3-edge.com/wordpress-plugins/w3-total-cache/ dramatically improve the speed and user experience of your site

 SEO/SEM Plugins:

Google Analyticator -http://wordpress.org/extend/plugins/google-analyticator/ - this is a plugin to make integrating Google Analytics into your WordPress site easy

Google XML Sitemaps - http://www.arnebrachhold.de/redir/sitemap-home/ - keep track of Google Analytics right from your Dashboard

Gd-star-rating – http://wordpress.org/extend/plugins/gd-star-rating/ – let your users rate your posts and pages with stars or thumbs up ratings

Permalink Finder - http://www.blogseye.com/ - never get a 404 page not found again. If you have restructured or moved your blog, this plugin will find the right post or page every time

Postie – http://wordpress.org/extend/plugins/postie/ – just write an email to post to your website (special email account must be set up)

SEO Ultimate – http://www.seodesignsolutions.com/wordpress-seo/ - this all-in-one SEO plugin gives you control over title tags, noindex/nofollow, meta tags, rich snippets, slugs, canonical tags, autolinks, 404 errors, rich snippets, and more

Slick Social Share Buttons - http://www.designchemical.com/blog/index.php/wordpress-plugins/wordpress-plugin-slick-social-share-buttons/ - floating social media buttons wherever you want them

Security Plugins:

It is imperative that you keep all your WordPress installations up to date, your plugins and themes as well.  Read about updates here.

Askimet – filters out spam and is automatically installed with WordPress

You must get an API Key from WordPress - http://en.wordpress.com/api-keys/ Sign up for a WordPress Account at http://www.wordpress.com After signing in to your account, Click on My Account in the upper left corner, select Edit Profile and at the top of that page you will see your Askimet Key Number. Copy and paste the number into your Askimet plugin

BackWPup – http://backwpup.com/ - easy backup to server or to your computer

Really Simple Captcha - http://wordpress.org/extend/plugins/really-simple-captcha/ - for use with Contact Form 7 to keep computer bots and spiders from collecting sending automatic spam to your email

WP htaccess Control - http://dardna.com/wp-htaccess-control - another security measure for WordPress - interface to customize the permalinks (author, category, archives and pagination) and htaccess file generated by WordPress

Better WP Security - http://bit51.com/software/better-wp-security/ - takes the best WordPress security features and techniques and combines them in a single plugin thereby ensuring that as many security holes as possible are patched  (can be used in conjunction with Wordfence Security and WordPressFirewall in Atahualpa Theme)

Wordfence Security - http://wordfence.com/ - anti-virus and firewall security plugin for WordPress (can be used in conjunction with Better WP Security and WordPress Firewall in Atahualpa Theme)

WordPress Firewall - http://www.seoegghead.com/software/wordpress-firewall.seo -blocks suspicious-looking requests to WordPress- sign up through their website to receive email alerts (can be used in conjunction with Wordfence Security and Better WP Security in Atahualpa Theme)

These plugins get my thumbs up!

WP-Print - http://lesterchan.net/portfolio/programming/php/

WP – Wunderground – http://wordpress.org/extend/plugins/wunderground/?topic_id=17754 – add a weather widget to your sidebar

Yet Another Related Posts Plugin -http://yarpp.org/