I also want to emphasize that Microsoft Excel and Google Docs perhaps play an equal, if not more important role in keyword research than any of these tools, because at the end of the day, the mountain of data you end up with is of no use if you can’t perform calculations, sort, filter, present or store it in a way that’s best suits the task at hand. Head over to Distilled for a comprehensive Excel for SEOs guide. Alternatively, bug this guy—he’s the Sensei who trains Excel ninjas in the dark of the night.
Long Tail Pro is a keyword research tool that helps you quickly find low competition keywords for your website. In particular, the tool is built for people doing SEO (search engine optimization) as opposed to people doing paid ads (Google Adwords PPC). I'm sure there are some advertisers that use the tool for paid ad keyword research, but the tool was built for SEO keyword research.