There are thousands of WordPress plugins that you can download it from WordPress plugins repository for free. Since there are many plugins to choose for, please be aware with the plugins compatibility. Not all plugins are good. Some of them are abandon project. I will share some list of plugins that might be useful for you to start your WordPress blog.
Akismet is spam filter thatn come by default with any WordPress installation. It keeps your site protected from spam even while you sleep. You need to signup for Akismet API and add it to 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 help you to do almost everything onsite SEO for your blog.
Google XML Sitemaps
This plugin allow you to generate xml sitemaps for your blog content. Which you can submit it to major search engines which allow them to indexed your blog easily.
Contact Form 7
A simple but good contact form system. You need to have contact page so your reader know how to contact you for further relation or probably business proposal. It doesn’t have complex feature, but it simple and flexible.
Wise man said, backup early and backup regularly, you don’t know when shit happens. In my 2 years blogging i have some major issue with my blog. Got hacked or all data wiped out by the provider is common disaster that happen from time to time. With this plugin allow you to scheduly backup your WordPress database and send it to your email as attachment. Very recommended plugin.
There are alternatives cache plugins for WordPress. The good plugins are Hyper Cache + DB Cache reloaded fix, W3 Total Cache and Wp Supercache. They have pros and cons. The most complete one is W3 Total Cache but require high resources if you have lot of posts (more than 50K posts). I like Hyper Cache + DB Cache reloaded fix, they cache data as it is need. Read more about WordPress optimization from my previous post.
Yet Another Related Posts Plugin
This plugin allow you to show related post to the visitors. This YARPP use fuzzy algorithm to find the most related post to your current post. Showing related post to your visitors allow them to spend more time in your website and read. Which is good for you and for your visitors.
There are many good plugins out there, but the above list of plugins is enough for you to start with. And it depends on what you need to achieve.