Can't go wrong with the Google Keyword tool, SEMRush and Google Trends. The Keyword tool for volume estimates, SEMRush to see what keywords competitors are ranking for/targeting, and Google Trends to make sure the traffic is actually coming from countries I'm trying to target. Gives a relatively accurate picture of when to expect traffic spikes and seasonality insight.
Before you can put it to use and start making Long Tail Pro pay for itself, you first need to understand exactly what Long Tail Pro does and how it will benefit you. The number one benefit and feature of Long Tail Pro is that it helps you analyze the competition in Google much faster and more effectively than if you tried to do it only using free tools.