Greatly improves your Search Engine Optimization. Install and activate it and read the guide here (long but worth it).
This comes with WordPress – activate it and add your API key, and protect yourself from spam.
With version 2.0 coming very soon, WooCommerce is the best shopping cart plugin I’ve used for WordPress, and it’s free. If you don’t need a shopping cart to sell physical or digital products or services then you of course don’t need this.
This speeds up your site by making static copies, but please, please refer to your web host for the best way to configure this. Usually it’s simple for shared hosting (page and browser cache on; database and object cache off; minify on if you know what you’re doing) and it can have a big effect on performance.
5) A Contact Form plugin
Having a contact form on your contact page not only looks professional but protects you from spammers. For free I like Contact Form 7, and for paid I love Gravity Forms (which I use on my own site). Gravity Forms has many benefits such as multi-page forms, email newsletter subscriptions, conditional fileds (if this is selected, show that) and so on.
BONUS 6) WordPress Editorial Calendar
This makes scheduling posts, seeing when posts are scheduled, and changing scheduled posts much easier. Simply shows a calendar with your blog posts (and times) marked on. For sites with multiple authors, or if you schedule a lot of posts, you’ll wonder how you lived without it.