Keyword Surfer - Get search volumes & keyword data directly in Google.
Ubersuggest - Want more traffic? Ubersuggest shows you how to win the game of SEO. Just type in a domain or a keyword to get started.
Keyword Explorer by Moz - Find the best keywords to improve your website's SEO traffic
Keywords Everywhere - Free browser add-on for keyword volume, CPC & competition
Content Marketing Stack - A curated directory of content marketing resources
cognitiveSEO - The cognitiveSEO tool provides a unique analysis process that delivers Unparalleled Backlink Analysis, Content Audit and Rank Tracking for Every Site.