1) Akismet: It is one of the best and must have plugin for your wordpress blog which is used by millions of people around the world. It protects your blog from comment & trackback spam.
It keeps your site protected from spam even while you sleep or not working on your site.
2) All in One SEO Pack: It Optimizes your WordPress blog for Search Engines (Search Engine Optimization).
Update: Best Alternative To ‘All in One SEO Pack’ – ‘WordPress SEO By Yoast’ Plugin
3) Google XML Sitemaps: This plugin will generate a special XML sitemap which will help search engines like Google, Yahoo, Bing, & Ask.com to better index your blog.
With this plugin, crawlers would be able to see the complete structure of your site & retrieve it more effectively. Also it notifies all major search engines every time you post new content on your blog.
Update: You should also use PubSubHubbub Plugin that notifies all major search engines in a much faster way(real time).
5) Strictly Auto Tags: Tired of finding tags for each post? You can use this plugin to auto generate tags on each post and get easy SEO. Though I would suggest you to tag post manually for better site links and infrastructure.
6) Facebook Comment: With more than 800 million people on Facebook, you can’t miss this plugin. Full Facebook Comments moderation & management for your WordPress site. Very easy and quick to set up.
Update: Try Facebook Official WordPress Plugin.
7) Contact Form: A super customizable contact form that lets your visitors send you email. Very easy to set up forms on any page or post.
8 ) The Slide by SimpleReach: One of the few plugins which is growing at a tremendous speed. It helps your readers to discover more content on your site and increasing users’ engagement.
Update: Best Alternative To Simplereach Slide – nrelate Flyout WordPress Plugin
9) WP About Author: Easily display customizable author bios below your posts.
10) User Photo: It Allows users to associate a photo with their account on wordpress site & this photo to be displayed in their posts & comments too.
11) Add Post URL: This plugin allows you to insert a specific text such as advertisements, copyright, etc. at the beginning and ending of your posts.
12) Yet Another Related Posts Plugin: It displays a list of related entries on your site & feeds based on a unique algorithm. Out of all the related post plugin I’ve used till now, this is the best related posts plugin.
13) Disqus: One of the most used commenting plugin for websites. It allows your visitors to choose their identity (Facebook Connect, OpenID, Twitter) when they leave a comment. You can use this plugin only in place of “Facebook Comment’ plugin explained above.
Read: Top 5 Best Commenting Systems For Your Blog
14) Shareaholic: One of the most used and best plugin for users to share your posts. You have the option to put it above or below the posts.
15) TF Social Share: Another must have plugin for your wordpress blog to give floating share options on the left side of the posts. One more great plugin to add vertical share box to the left of the post is Sharebar.
W3 Total Cache: This plugin improves your site performance & user experience via caching: browser, object, database, page, minify & content delivery network support.
Update: Now, I am using WP Super Cache. to improve site performance as I found it lighter than W3 Total Cache.
Auto Post Thumbnail: It automatically generates the Post Thumbnail (Featured Thumbnail) from the first image in any post if Post Thumbnail(Featured Image) is not set.