Tools that can help you with content marketing are:
For topic and keyword research, content audits, content creation and more.

This is my favorite tool for content marketing and the tool I use on a daily basis for all digital marketing tasks. It’s not free, it comes with a monthly subscription but worth’s every cent.
For content marketing, you can use SEMRUSH to do topic research, keyword research, and competitor analysis.
If you don’t already have a subscription with an SEM tool, then SEMRUSH is the way to go.
Before selecting SEMRUSH, I have tested a number of other tools but none of them has the functionality of SEMRUSH at this price range.
You can register for a 30-Day free trial and test it yourself.
For monitoring the performance of your content

Google Analytics is a free tool by Google. You can use it to monitor the performance of your content marketing campaigns and to understand how users interact with your website.
For monitoring the Google rankings of your content

While the primary function of Google search console is to optimize your website for technical SEO, it’s the tool to use to optimize your pages after they are published.
Keyword research is an on-going process and the best tool to use to find out the actual keywords your pages are ranking is the Google Search console.
For topic research

As explained above, another tool you can use to find out about new and trending keywords is Google Trends.
Go to Google Trends and perform several searches, using different filters to find out what is trending in your industry.
For keyword research

The Google keyword tool is a free tool provided by Google that allows you to do keyword research for PPC (Google Ads) purposes but you can also use it to do topic research for content marketing.
The tool is free but in order to use it, you need to have an active Google Ads account.
Go to Google Ads and register for a free account. Then, create a Google Ads campaign but do not activate it, keep it paused and access the tool by selecting TOOLS and then KEYWORD PLANNER.
For reporting and analysis.

Google Data Studio is a free reporting tool by Google. You can use it to combine data from different sources (including Google search console and Google analytics) and create beautiful reports to monitor the performance of your content marketing campaigns.
For improving the grammar and spelling of your text content.

Grammarly will help you write content that is free of spelling and grammar mistakes.
KEY LEARNINGS
These are the tools for you need for your content marketing campaigns: