Doing research on this right now. Given the vast majority of free tools have query limitations. The following appears at present to be the strongest solutions:
free - https://freekeywords.wordtracker.com/searches, http://soovle.com, http://ubersuggest.org, http://keywordsuggest.org/, http://www.google.com/trends/
free & paid solution - http://www.keywordeye.com $9.99 pro per month
free 10 day trial - longtailpro.com
quality paid solutions - http://www.semrush.com $79.95 per month, http://www.wordtracker.com $69 per month,
Some of the paid services functionality overlap moz.com so it would not be worth paying just for just a keyword suggestion/scraper tool in my opinion.
