Google Adwords
Get monthly keyword volume data and stats.
Google Trends
See how many users are searching for specific keywords.
Keyword Discovery
www.keyworddiscovery.com
Keyword Spy
www.keywordspy.com
KeywordTool
keywordtool.io
One Look Reverse Dictionary
www.onelook.com
Soovle
www.soovle.com
Ubersuggest
ubersuggest.org
Word Tracker
www.wordtracker.com
Yandex Wordstat
wordstat.yandex.com