I use Wordpress on most of my sites and have found myself installing the same few plugins over and over again because they're just so useful. The following is a list of plugins that will make your life easier if you're using Wordpress. Note: As of writing, these plugins work with Wordpress 3.5.1
Useful Wordpress Plugins and What They Do
WordPress SEO by Yoast
I have this on every site! The plugin helps optimize SEO for your site and adds a section below your post/page editor that allows you to enter a target keyword. It'll show you an analysis of the page on a different tab based on the keyword. Below is an example:
Google Analytics for WordPress
If you want to set up Google Analytics for your Wordpress site but don't want to have to add the code in every page or edit the template files or whatever, just enter your UA code in this plugin and it'll do the rest for you.
Login Security Solution
I got this one recently because I've seen a lot of complaints about Wordpress sites being hacked. Login Security Solution helps slow down brute force attacks and sends you an email when your site is under attack. I didn't think I would need this a day after I installed it, I received an e-mail from the plugin reporting that someone has attempted to make 50 failed login attempts!
This adds a lot of useful options in the editor that lets you change your fonts, add headings and a lot of other stuff that are not available in the original editor.
W3 Total Cache
Makes your site run faster :) The video below shows you how to set it up on your Wordpress site
This plugin adds a checkbox that allows you to make nofollow links easily. I installed the plugin and forgot about it, only realizing how useful it was when I created a new site and couldn't find the checkbox.
Remove Image Links
By default, Wordpress links any images in your posts directly to the image file. That's not very useful unless it's actually an image that you want your readers to go to. Most of the images on my site are for making the post prettier and I don't really want readers clicking on them to go anywhere. Instead of removing the links one by one (too many images!), I use the Remove Image Links plugin which does that automatically.
I have this because I wanted something like the references you can add to the bottom of InfoBarrel articlces. WP-Footnotes is the closest thing I found to that. This is useful if you have articles on your site that require a lot of references but you don't want the entire article to be cluttered with external links.
Hupso Share Buttons for Twitter, Facebook & Google+
Adds a toolbar to the top, bottom, or both top and bottom of your posts so readers can easily share your posts to social networks. Besides Twitter, Facebook, and G+, it also has buttons for Pinterest, StumbleUpon, LinkedIn and more. You can customize the size of the toolbar and which social network buttons you want in the toolbar.
nrelate Related Content
This automatically adds other related pages or posts on your site to the bottom of your content. It probably isn't very useful until you have lots of stuff on your site.
Don't add them all at once!
While the above plugins work pretty well with each other (I haven't had any problems), you may have other plugins that might not work well together so well. If you're installing a set of plugins for the first time, add them to Wordpress one by one and check if anything breaks (rare, though) before installing the next one.
That's it! If I left out anything or if you have any questions or comments, feel free to leave a message below :)