It’s said that these here below are the 3 most important SEO tools:

Backlink Checker is where you’ll be able to find the web sites that will link back to yours, you’ll also be able to see the text, the page’s Page Rank, if they use the “nofollow tag”, etc.

Niche Watch It’s a free tool, it’ll tell you which sites are on the front pages of the search engines, depending on the words you’re using.

Page Rank Checker Even when many people don’t think this one here is important, I still feel it’s important to keep the pagerank in mind.

Source: What is SEO?