As almost everyone has said, LongTailPro is a good choice. BUT, you don't need it. I do most of my research using the SEO Quake toolbar and simply analyzing the first page. Do they have the keywords in their title, URL, description? If so, lots of people are gunning for that keyword. If not, not as many people are targeting it. The top 5 is where you should focus most of your attention. I analyze them using SEO Quake and a quick visual look over, then copy/paste into Market Samurai to analyze how many links they have to that page. Identifying authority sites that rank based on authority versus links is important too. They're harder to outrank.