I use a few different keywords tools
this is a great free one
http://tools.seobook.com/general/keyword/#results
Google has one at
https://adwords.google.com/select/KeywordToolExternal
(google has great free instructional videos about adwords if you are going to use adwords learn a lot it first and especially about "keyword relevence")
Google adwords help center
https://adwords.google.com/support/?hl=en_US
I have used "Niche Inspector" which is pretty good.
Recently I got "Keyword Elite" I am pretty impressed. It will dig really deep across multiple search engines. (to get littte used cheaper and less competative keywords)
nichbot.com is a good online keyword search tool, monthly membership, it's interface is a little complicated but it does a huge amount of search and will return results in many ways and has filters to narrow to the best useable terms.
There are some related just to adwords, like Adwords Accelorator, I kind of liked it but maybe his next version will be more worth it.
There are a few other big ones out there, if you search and find others I recomend you find some real reviews about them (not just affiliates with phoney review sites). the Warrior forum has a product review section.