I often use my hoster's analytics tools. I get the related keywords (and the keyword of my choice) and the number of hits per month.
there are various tools available to search keywords like google adwords, keyword playground,keyword suggestion tool etc...try them...
I already knew about google adwords. Just tried keyword playground and will be checking out keyword suggestion tool. Any others? Thanks.
market samurai is the best. It very completely because we can chek too our competitor too. If you want, try free acces firstly
Definitely Google trends. And if it's social advertising concerned, Twitter trends as well. The language of today's news seems to reflect how much of a story is trending, rather than the story itself.