KeywordTool.io - KeywordTool.io is the best FREE alternative to Google Keyword Planner and Ubersuggest. It uses Google's autocomplete feature to get over 750+ long-tail keywords for any given query.
GrepWords - Grepwords is an API-first Google Keyword Suggestion Tool.
Google Trends - Explore Google trending search topics with Google Trends.
SheerSEO - SheerSEO helps you to track Google Search Engine Rankings for critical keywords. PageRank, Back Links and more.
Keywords Everywhere - Free browser add-on for keyword volume, CPC & competition
iSpionage - AdWords competitor keywords with the click of a button and monitor competitor PPC landing pages.