Researching keywords for campaigns is often the most annoying part for online marketers. There are tons of tools out there, but only a few really give you what you need. Especially with SE changes like Google's quality score it is necessary to be smart when it comes to researching and buying keywords.
My favorite tool is "Keyword Curious", developed by my friend Scott, who is one of the best
Keyword SEO Wordpress tool developers out there (I am not exaggerating! Just check his Domain Hunter or Typo Squatter tool).
So check it out...and yes it is free.