For Amazon FBA/Reselling? Or for Affiliate Marketing? If it's for selling physical products as a third party merchant Keyword Inspector is awesome (mainly if you are going to be doing private label selling). It lets you see what keywords top selling products are ranking for in Amazon. You can then use target your efforts toward those keywords. There's also merchant words that gives you good ideas as to what people are searching for. It could probably be used for Amazon associates research too. It all depends on what you are going to be selling on Amazon or if you are doing it for Amazon associates.