SEO Tool

Google Trends

Google trnds help to perform keywords research by the trending searches

http://google.com/trends

Google Adwords Keyword Planner

A free Google Adwords Tool, Keyword Planner to get ideas, search volume and keyword traffic.

https://adwords.google.co.in/KeywordPlanner

Robots.txt Tester

Robots.txt testing tool in serch console for testing the robots.txt file

https://www.google.com/webmasters/tools/robots-testing-tool 

HTML/XHTML Validator

W3C Validator Check markup validity of Website documents.

http://validator.w3.org/

Search Console (Google Webmaster tool)

It's a free service offered by Google that helps monitor and maintain presense of site in Google Search results.

https://www.google.com/webmasters/ 

Google Analytics

Its a free services offered by Google to track your website by code and GA helps you analyze visitor traffic.

https://www.google.com/analytics/