Google Trends

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If you have your own Blog, which does concentrate on one particular subject, then you may want to look at Google Insights For Search.
Let's say for example, you have a Blog about Recipes and Cooking tips, wouldn't you like a few more hundred visitors a day, and visitors, could mean money for you, if you monetize your Blogs.

It's important to start with your main phrase first, just typing in "Recipes", this will help you see the Rising Searches for every type of Recipe. Today for instance(01-27), the highest rising search for the past week, has been "Australia Day Recipes" and "Super Bowl Recipes", a market, that may be possible to get into. The #6 Rising Search is Green Bean Recipes, a smaller niche, popular, and it has very little exact match terms on Google.

This is just another way, of narrowing down niches with Google Insights For Search.