In no particular order:
Buzzsumo comes in a free and paid version. The free version allows you to get valuable insights about your influencers.
You can see who shared your content, who engaged with it on sites like Facebook and Pinterest. Even though the amount of information in the free version is limited, it can help you to understand who is engaging with your content and which of your articles and photographs have the biggest impact.
I learned a lot about my readers, if you had asked me before using Buzzsumo I would have guessed that other articles are customer favorites.
You might already know Jeff Bullas´ tool set. Latest when you look at the site, you will recognize that you see his SumoMe on several (leading) websites.
Besides that they will try to get you to upgrade, most functions are accessible free of charge. The screenshot shows a few features.
Traffic TravisTraffic Travis is a pretty universal tool. SEO, Keyword Research, Competitor Research, Backlink Check, Page Analysis and much, much more.
In my opinion, Traffic Travis is not the No. 1 in any of this categories but that you get all of the functions in one tool for FREE is amazing and in a way makes it "no 1". They also offer a "Professional" version that, in all honesty, I have not yet tried.
Check out this video to learn more: Traffic Travis