The Keyword Difficulty Check tool will guide you on your way. Just type in a keyword or phrase, and it lets you know how easy or difficult it would be to rank for that term. Get high rankings for less-common keywords, or at least know what you're getting into with more popular words.
I found this tool incredibly easy to use, and yes, it's FREE. Give it a try. Simply navigate to the URL noted above and type in a keyword phrase. This is a great place to start before choosing your keywords or evaluating the competitiveness of the keywords you're optimizing for. Enjoy!