1. Google Keyword Planner with MS Excel.
2. Market Samurai
3. Long Tail Pro
4. Keyword discovery
You could also try out Keyword Revealer and they do have a free plan as well as paid. They let you see google insights, moz and social metrics and domain availability too. Hope this helps you out in making your decision.
Best response so far.
People need to remember though, data that is displayed via keyword planner is specifically for the purpose of advertising, not SEO. Some of the best keywords will not be displayed.
Market Samurai and Long Tail Pro are great options as well (never heard of keyword discovery). Both those tools tie into the keyword planner.
One of my favorite tools just recently discovered is: answerthepublic dot com(sorry, not enough posts to share a link)
It doesn't show statistics or any kind of metrics. It does give you an idea for content you could create though, and supporting headings, particularly useful for local SEO but even on a broader scale can be very useful, and it's free.