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.
Keywords Everywhere - Free browser add-on for keyword volume, CPC & competition
Keyword Tool & Content Assistant - Content optimization tool to boost your rankings
Keyword Explorer by Moz - Find the best keywords to improve your website's SEO traffic
Keyword Chef - Simple keyword discovery tool built for publishers