There are thousands of plugins in the WordPress Plugin directory. These are 13 plugins I seem to revisit the most often and recommend to clients and friends. This list differs from the one I wrote a little over a year ago because a) it has pictures, and b) a lot has happened since last year.
Gravity Forms
Gravity forms makes it incredibly easy to create web forms for your site. Some things I liked about it were the conditional form fields and the ability to monitor and export data. If you can’t shell out the money for Gravity Forms, Contact Forms 7 is still a suitable alternative.
Google Analytics for WordPress
As far as Google Analytics plugins go, this one is the best. Not only does it assist in adding the Google Analytics tracking code but it can also track outbound links and insert itself in the header or footer.
WordPress SEO
WordPress SEO is the most complete search engine optimization plugin you will find. Not only does it assist with titles and meta information but it can create XML sitemaps, breadcrumbs and much more.
Open Graph Protocol Tools
This plugin adds meta information to your WordPress header to enable your web page to become a rich object in a social graph. The Open Graph Protocal is used on Facebook to enable any web page to have the same functionality as a Facebook Page. If you’re having problems with Facebook or Digg using the wrong image when you try and submit your posts this plugin might help.
Socialize
I have a few plugins in the WordPress plugin repository but this is the only one I think people really need to know about. It allows you to selectively display the most popular social bookmarking buttons on your website. It also provides a call to action area at the bottom that you can edit for specific posts to request further action from your readers.
W3 Total Cache
W3 Total Cache is the fastest caching plugin for WordPress and is used by a lot of popular websites.
WPtouch
WPtouch is the most popular plugin for making your website mobile ready. The plugin detects mobile browsers and styles your site accordingly.
Secure WordPress
Secure WordPress allows you to selectively turn off WordPress functionality that poses a threat to your site.
Yet Another Related Posts Plugin

A great way to keep people on your blog is to recommend similar blog posts when they have finished reading the current one. The plugin has a bunch of settings making it easy to customize for your blog.
WP-DB-Backup

This one is straight and to the point. Back up your wordpress site so if something goes wrong you don’t kill yourself.
Subscribe to Comments

This plugin goes on my list of essentials. It’s the easiest way to get repeat visits from your commenters. It basically allows commenters to subscribe to all future comments on a post.
WordPress Blog Stats

I use this in conjunction with Google Analytics for two reasons.
- I like having stats integrated into my dashboard.
- I like being able to use that data with other plugins.
It’s not like the data is collected any faster than Google Analytics as both have a decent delay however it’s nice to see what another service has to say about my websites traffic.
Akismet

This one’s simple, it marks spam as spam for you so you don’t have to.