When I build WordPress sites for my clients (or heck, for myself), I have a set of plugins I tend to fall back on for increased functionality and awesomeness. While I'm a happy WordPress custom theme developer, I don't tend to do much with plugins, save install them and sometimes modify them to the client's needs. I figure, my main focus is on theme creation, so let someone else focus on great plugins.
Just kidding! But it is probably better to let the great plugin devs do their work, and not ask your dog to...
If you're new to WordPress, building your own site for the first time, or just need a bit of a refresh, here are some of my favourite (free!) plugins, in no particular order.
Nextgen: Need a photo gallery for your site? Nextgen is a fantastic option. The free version offers a number of different ways to display your photos, including sliders and several types of lightboxes. An easy-to-use image management system is included, and I love the fact that I can upload an entire zip file of photos, which it will extract for me. If you need more fancy features, such as a fullscreen lightbox or e-commerce to sell your photos (or other work), they have two levels of paid versions as well.
WP Supercache: For newer folks, a caching plugin, in a nutshell, helps speed up your site by 'saving' assets so a user doesn't have to download them every single time they go to the page. For more on this, the folks at wpmudev wrote this post about caching - give it a read if you like.
There are a few different caching plugins available, and this one is my favourite. It's very simple to use, especially for folks who don't have much technical background.
Contact Form 7: There are a number of good contact form plugins out there but I prefer this one because it is pretty plain-looking to begin with. This means I can easily do a lot of styling without modifying anything in the plugin itself. As well, it offers a lot of options for types of contact fields (text, checkboxes, email, phone, etc), and includes CAPTCHA (if you do a few extra, easy steps), an important feature for decreasing spam messages.
Duplicator: I learned about this one when developing my first WordPress theme for a client and instantly fell in love. Now mind you, I am more technical than the average WordPress user, and the creators of the plugin do warn you that it is intended for more advanced users. In order to use it, you need to be comfortable creating SQL databases, but in the vast majority of cases your web host will provide an easy-to-use interface to do so.
If you want to literally clone your site and bring it somewhere else, like to a new URL or from your local machine to your web host, this does the trick in a super easy way.
AccessPress Social Icons: This was a recent discovery working a client's site. Super simple plugin that allows you to display social media icons in an attractive way just about anywhere on your site using shortcodes, php in your template, or a widget. Given how social media is so important to most businesses these days, this is a good plugin to check out.
WordPress SEO by Yoast: I think of this as the granddaddy of all SEO plugins. WordPress SEO is a huge topic to itself, and honestly, you need a plugin to help you manage this. They've done a great job of writing this plugin and writing a ton of documentation to go with it. Read their documentation on SEO for WordPress sites and you'll quickly see they know their stuff.
For devs or more advanced users:
Custom Post Type UI & Advanced Custom Fields are two great plugins for WordPress theme developers. They essentially do as described: Custom Post Type UI creates custom post types for your site, and ACF lets you create custom fields of pretty much any kind to add to posts or pages. For example, you could add a post type of 'Testimonials' and then add in fields for those testimonials, such as name, email, and the testimonial itself. Read the docs for more info if you're doing some theme development and want to use them.
And last but not least, an honourable mention goes to the awesomeness that is Cloudflare. This isn't a plugin but it's a good way to optimize and 'speed up' your site. A good number of web hosts include easy integration with Cloudflare these days, so it's almost a no-brainer to get it running.
So how about you guys? What are your favourite plugins or WordPress add-ons?