Aaron Wall has a nice write up on many of the keyword research tools available today. He runs down some of the pros and cons with each, as well as gives you a list of items you should be aware with for any of these tools. Aaron explains that you always need to be concerned with "the biases of the providers" because they might have a ventured interest in you purchasing a specific keyword or two. He also explains that all the figures are "estimates" and to "consider how spread out the search terms likely are in your industry." Aaron also posted at ThreadWatch that WordTracker is selling top 500,000 keywords it has collected over the years.
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