You can use Google Adwords Keyword Planner tool free.
There is no perfect tool. Combine functionality of the following:
Google keyword planner
Google Insights
Google Trends
Traffic Travis Free
UberSuggest
LSIkeywords dot com
Google Analytics/Webmaster Keyword Suggestions
Wordtracker
Wordpot
Also, check
Social media hash tags/popular terms
Amazon/Clickbank best sellers
Related forums for actual terms used by actual users
Competitor blogs and the terms they use
... anything that has something to do with the customers and your competitors... it is a free keyword research tool.
Google Adwords Keyword Suggestion tool is now changed into Google Keyword Planner. To use it you have register for Adwords. Registration is free.
Some other good keyword research tools are:
SEMRush: http://www.semrush.com/
Keyword Eye: http://www.keywordeye.com/
SpyFu: http://www.spyfu.com/
Keyword Spy: http://www.keywordspy.com/
Just to add to the other comments - there are loads of other tools that you don't need to pay monthly subscriptions for , but you can just do a small one-off fee for. My favourite tool like this is Keyword Snatcher, and it's become the cornerstone of my keyword research.
I'm actually running a giveaway of a Keyword Snatcher license right now on my blog if you're interested, plus I've outlined a full tutorial on how I carry out extensive keyword research - http://findmyblogway.com/keyword-analysi...
Google keyword planner is the best tool for the Keyword analysis and search.
I use Google Adwords Keyword Planner + Trafic Travis (Free Version).
Since google keyword is no more ._.
