Google Adwords Keyword Tool is a favorite tool for sure — it is free and powerful enough for many purposes. It is the first (and only) keyword research tool that many people use who don’t want to pony up and pay for an even more powerful tool.
It is easy and fast. Here is an example I performed for Catalina Vacations which offers vacation rentals on Catalina Island:
Check out the SEMRush tool for awesome keyword insights based on phrases your competitors are using succesfully.