SEOMoz Keyword Difficulty Tool
SEOMoz offers a useful tool for assessing the difficulty of achieving a high ranking on a particular keyword. It examines both on-page factors (keyword density, title and alt tags, etc.) and in-bound links, providing a slew of details and an overall “difficulty score”. While we primarily use our internal techniques and tools, we occasionally look for tools like this that offer additional capabilities and insight.
Note that the Keyword Difficulty Tool requires Google and Yahoo API codes. Links are provided to the appropriate registration pages.