Using the Google adwords tool as suggested,its free and all the other paid tools like wordtracker market samuri etc pull their data from here, but when you are in the adwords tool use the the website section and type in a domain that is at the top of the search engines in your niche, and Google will give you all the keywords that that site is ranking for.
If you're looking for keywords your business should be tracking, RankZen will help you identify which keywords your potential customers are searching for. Other features include keyword research reports, tracking the position of your URL compared to competitors, and discovering missed keywords. You'll save money, time, and discover the true essence of your industry in the world wide web.