This tool was originally created to carry out keyword research for paid campaigns using Google's AdWords platform. However, it can also be used to research organic keywords, providing estimated global/local monthly search volumes for keywords (i.e. their popularity) and their competition (i.e. how difficult they are to rank for) for organic search.
In a nutshell, Long Tail Pro helps you quickly find keywords in bulk based on a seed keyword that you input. In addition to returning hundreds of related keywords, the tool also shows search volume (monthly search volume), advertiser bid, number of words, rank value, and my favorite Keyword Competitiveness (helps you judge the competition and keyword difficulty).