There are many tools that can be used to look for keywords, but Google's own keywords research tool is often recommended: it's free, and no one knows more about key-words than Google does.
The tool is provided as part of the AdWords product (ref AdSense vs AdWords what's the difference).
Usually, you need to sign in to an AdWords account to use their tools. An AdWords account may be based on your usual Google account, but because it can be used to buy advertising, Google ask you to put a small amount of money into it, so you can use it to pay for advertising campaigns immediately. (They don't charge you immediately, they just want your account to have a positive balance, so it's able to be used.)
However their Search-based Keywords tool (announced back in 2009) and it's upgraded version, the Keywords-tool ( http://www.adwords.google.com/keywordtool or https://adwords.google.com/select/KeywordToolExternal - both which currently re-direct to https://adwords.google.com/o/Targeting/Explorer?__c=1000000000&__u=1000000000&ideaRequestType=KEYWORD_IDEAS ) have been available without logging in, provided you complete a simple captcha-text .
Today, though, I've noticed this announcement at the top of the page:
In the coming months, the external Keyword Tool will no longer be available. To get keyword ideas, sign in to your AdWords account and try Keyword Planner.
I cannot see any announcements about this in the AdWords blog. But this actually has very little information about the keywords tool anyway, most of what is there dates from 2008/09, and they haven't used "keyword" as a label for their posts.
Google aren't in the habit of putting up notices like that which are wrong, so my guess is that, sooner or later, this tool is going to be removed, and their only keywords-research option will be the Keyword Planner, which isn't free .
So maybe it's time to start finding a new favourite free keyword research tool. Some reviews to get started with:
Or maybe bloggers should just forget about keyword research, and focus SEO-basics, and then on writing good quality, natural-language, unique content.
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Thanks for reading: AdWords external keywords research tool is going to be retired, Sorry, my English is bad:)