There are a number of free effective SEO tools available for keyword research. However, the best one to use depends on your specific needs and goals.
One popular free effective SEO tool is Google AdWords Keyword Planner. This tool allows you to research keyword phrases and get estimates for how much traffic they could generate.
Another free effective SEO tool is Google Trends. This tool lets you see how popular a particular keyword phrase is and how it has changed over time.
If you're looking for more targeted keyword research, consider using a paid tool such as WordTracker or KeywordSpy. These tools provide more in-depth data and analysis, but they come at a price.
Finally, don't forget to use your own common sense and experience when conducting keyword research. No tool can replace the value of human intelligence and intuition.