There are thousands of WordPress plugins that you can download from the WordPress plugins repository for free. Since there are so many plugins to choose from, please be aware of plugin compatibility. Not all plugins are good. Some of them are abandoned projects. I will share a list of plugins that might be useful to start your WordPress blog.
Akismet
Akismet is a spam filter that comes by default with any WordPress installation. It keeps your site protected from spam even while you sleep. You need to sign up for an Akismet API key and add it to the Akismet configuration.
All In One SEO
This is a must-have WordPress plugin. All-in-One SEO Pack allows you to add unique meta description, keywords, and titles to each page of your blog and blog posts. All-in-One SEO Pack helps you do almost everything for on-site SEO for your blog.
Google XML Sitemaps
This plugin allows you to generate XML sitemaps for your blog content, which you can submit to major search engines to help them index your blog easily.
Contact Form 7
A simple but good contact form system. You need a contact page so your readers know how to contact you for further relations or possibly a business proposal. It doesn’t have complex features, but it is simple and flexible.
WP-DBManager
Wise men say backup early and backup regularly — you don’t know when things go wrong. In my 2 years of blogging I have had some major issues with my blog. Getting hacked or having all data wiped out by the provider is a common disaster that happens from time to time. This plugin lets you schedule WordPress database backups and have them sent to your email as an attachment. Very recommended.
Cache Plugins
There are several cache plugin alternatives for WordPress. The good ones are Hyper Cache + DB Cache Reloaded Fix, W3 Total Cache and WP Super Cache. They all have pros and cons. The most complete is W3 Total Cache but it requires high resources if you have a lot of posts (more than 50K posts). I like Hyper Cache + DB Cache Reloaded Fix — they cache data as needed. Read more about WordPress optimization in my previous post.
Yet Another Related Posts Plugin
This plugin allows you to show related posts to visitors. YARPP uses a fuzzy algorithm to find the most related post to your current post. Showing related posts lets visitors spend more time on your website reading, which is good for everyone.
There are many good plugins out there, but the above list is enough to get you started. It depends on what you need to achieve.