There are many keyword selection tools and avenues to explore. Some are free, while others require a subscription. The most important part of determining your keywords is getting your arms wrapped around the mind of your customers and prospects -- what terms will they use? Many times we are too close to our products and services, and we've got all the buzzwords that seem second-nature to us. Be careful to check your perspective at the door, and get into your target market's head when determining the best keywords to focus on.
Below are links to some really good eBooks, tools, and blogs that help you work through this aspect. I recommend the keyword tools available - and we've got a list of them for you to check out.
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The keyword tools listed below help and are necessary to selecting the correct keywords for your SEO and/or SEM work. They all offer slightly different benefits, and most are not free. Which ever you arrive at using, they are definitely worth the investment.