I didn't saw any such thread and there were few questions asking this so thought to create a thread listing list of Free major keyword research tools - Free Keyword Research Software and Tools TrafficTravis Google Trends Bing Ads Intelligence Youtube Keyword Tools Bing Keyword Tool Google Keyword Planner Soovle Google Correlate You can post more such tools as reply and I will try to add them in the list EDIT - I can't post the urls unfortunately, so here are the urls in text format http://www.traffictravis.com/ http://www.google.com/trends/ http://advertise.bingads.microsoft.com/en-us/bing-ads-intelligence https://www.youtube.com/keyword_tool http://www.bing.com/toolbox/keywords http://adwords.google.com/ko/KeywordPlanner/Home http://www.soovle.com/ http://www.google.com/trends/correlate/
Some keyword research tools are given on these pages too: 1) http://www.1stwebdesigner.com/design/20-free-keyword-research-tools-comprehensive-insight/ 2) http://www.iacquire.com/blog/the-best-free-premium-keyword-research-tools Hope this helps.
Try Hittail keyword tool, it is one of the best out there. The tool not only suggests keywords but also gives a choice to get articles written for them too!
Free Keyword Research Tools: Bing Keyword Tools Google Webmaster Tool Youtube Keyword Google keyword Planner