It is not because a tool is not popular that it is not useful.
Most of the most useful free SEO tools tend to fall under the radar.For example, two of my favorite tools of all time are gInfinity (Chrome extension) and Chris Ainsworth’s SERPs extraction bookmarklet.
By combining these two free tools, you can extract several pages of SERPs (with meta titles + descriptions) in a few seconds. But these tools are rarely ever described by other SEOs.
Here are 30 more of the best free SEO tools (in no particular order):
Keyword Mixer – Combine your existing keywords in different ways to try to find better alternatives. Also useful for removing duplicates from your keyword list.
Note: MergeWords performs (almost) exactly the same job with a cleaner user interface. However, there is no option to delete the list.
Keyword Shitter – Generates hundreds and thousands of keyword suggestions from your basic keywords. You can also download all keyword suggestions into a .txt file in one click (no registration required).
LSIgraph.com – LSI (Latent Semantic Indexing) Keyword Builder. Enter a boot keyword, which will generate a list of LSI keywords (keywords and topics semantically related to your boot keyword).
TextOptimizer is another very similar tool that does pretty much the same job.
Keyword Search Tool SERPs.com – Finds thousands of keywords in seconds. It also allows you to filter these keywords and export them to a CSV file (no registration required). Note: Here is another similar tool from SEOBook.
Small SEO Tools Plagiarism Checker – Detects plagiarism by scanning billions of documents on the web. Useful to find those who have stolen / copied your work without attribution.
Google SERP Snippet Optimization Tool: Identifies truncation problems with meta titles and descriptions. Also generates a live preview of how your web pages are currently appearing in Google’s search results (this SEOMofo.com tool does the same thing).
Note: The CoSchedule Title Analyzer also shows how your title tag is displayed in the SERPs, and gives you tips on how to improve it and get more clicks.
Bulk Webpage Word Counter (by SEO Review Tools) – Does what he says about tin; mass checks number of words for up to 10 URLs.
SEO Hero – Find all the concepts, entities and N-Gram related to your subject or keyword. Or, in short, he discovers the “LSI keywords” that you can add to your content to improve relevance.
iSearchFrom.com – Replicate a Google search utilizing any location, device, or language. You can personalize almost everything from SafeSearch settings to custom search.
Delim.co – Converts a comma delimited list (ie CSV) in seconds. Not necessarily an SEO tool in itself, but certainly very useful for many tasks related to SEO.
Drop My Link – Combines your keywords with advanced Google search operators to discover link building opportunities. Very useful for finding guest posting opportunities, link / resource pages, link summaries, etc.
Keyworddit – Extracts the most frequently used keywords and phrases from any subprogram. It also shows the monthly search volume for each keyword.
What does my site cost? – tells you how much it costs to visit your website on mobile networks around the world. Very insightful for people accustomed to unlimited web browsing at 50 Mbps + 4G (like me!)
Keep alive Validation Tool – Checks whether a website (or a list of websites) supports a permanent connection. Those who do it are generally faster to load than those who do not.
Am I responsive? – Check the responsiveness of the website by showing you what it looks like on a desktop computer, a laptop, a tablet and a mobile.
HEADMasterSEO – Simple block status code (for example, 200, 301, 404, etc.) and a redirection checker. Also checks response time, response headers, and HTTP header fields (X-Robot tag, link, variable). Includes filtering and a lot of other advanced features.
SERPLab – Free Google Ranking Checker. Updates up to 50 keywords every 24 hours (if the server allows it).
Varvy – Checks if a web page follows Google’s guidelines. If your website is missing, it tells you what needs fixing.
JSON-LD Schema Generator – JSON-LD schema markup builder. It currently supports six types of tags, namely: product, local company, event, and organization.
KnowEm Social Media Optimizer – Analyzes your webpage to see if it’s optimized for social sharing. It checks the tags of Facebook, Google+, Twitter and LinkedIn.
Wheregoes? – Shows you the full path of meta-refreshments and redirects for any URL. Very useful for diagnosing link problems (for example, complex redirect strings).
Google Business Review link generator – Creates a direct link to your Google Business list. You can select among a link to all current Google reviews or a pre-filled 5-star opinion box.
Search and extract Twitter users Also useful for finding influencers who have shared content from a particular domain. Or about a particular subject.
Note: Here is another interesting tool that analyzes the latest 2K tweets for any Twitter account to determine who influences this person.