There are a lot of free SEO tools for executing simple SEO activities, e.g. building simple sitemaps, optimizing layered navigation, running basic SEO site audit, etc. You may find all of them on Magento Connect: https://community.magento.com
As you can see, the most popular SEO plugin is the one by Creare.
But note that some of the free Magento SEO modules may be of a low quality and do not work as expected. So we recommend you to read the reviews and check the demos first.
Definitely check out FREE module from Mageplaza: Magento 2 SEO module. It has many great features which cover up virtually every SEO aspects. Meta title, meta description, duplicated content, rich snippets, image alt tags, ... Pretty much everything. However, the SEO Checklist is mediocre and do not provide much useful information. Other than that, this extension is worth trying.
Your Magento 2 powered online store will function better than ever with the Free Magento extensions. These extensions promise improve SEO as well as efficient product delivery and customer services
These free Magento 2 extensions are meant to show the quality of our paid modules.
FREE Shop by Brands Magento 2
FREE Store Locator Magento 2
Free Magento 2 Banner
Free Blog For Magento 2
FREE Popup for Magento 2
Free Catalog ajax scroll Magento 2
From solving duplicate content issues to SEO templates, this extension knows everything.
SEO Suite For Magento 2 This extension comes is two versions - Basic and Pro. Both the extensions are providing different features.
All of the basic SEO features are covered by default Magento. So most likely you won't find a tool that allows you to configure much more for free or those will be separate solutions for canonical tags, cross-links HTML sitemap, and all of the other vital options for search engine optimization.
Instead, you can explore the benefits of the SEO Suite Ultimate Extension module for Magento 2 that offers you to configure all of the most important SEO parameters in one place, with just a few clicks.