For me google keywords tool is the best tool.
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The Keyword Research Tool will help you research appropriate words and phrases to include in your webpage's body text to aid in promotion. Simply enter the sort of word of phrase you wish to be found under, and the tool will suggest some additional words and phrases you can think about using.
There are numerous free tools available on net such as Google keyword tool, niche bot, overture keyword selector tool, pixel fast, digital pint’s keyword suggestion tool but the best tool which I like is Google keyword tool.
Google Adwords Keyword Tool is great tool to gather search query data. Keep in mind that the search volume is just an estimated number, but a good indicator nonetheless. SEMRush is paid tool but allows you to check which keywords your site (or competitors' sites) are currently being ranked for in the search results page.
See which keywords are on the cusp of breaking first page placements and optimize on page as well as do some link building to see if that would push it up further in rank.
why not just use the Google Webmaster Tool,its very useful for you to find the keywords.