This is my new favorite webmaster tool.

Enter the URL of the website you want to optimize for and then put in a target keyword. The results will show you two lists: the first will show you keywords and phrases related to your site and the second will show you keywords and phrases related to the keywords/phrase you entered.

Both lists will show you the monthly searches, the level of competition, and the suggested bid price (in Adwords) for each phrase. The first list will also show you the URL's where their suggested keywords came from (in your own site.)

This webmaster tool is very helpful in determining which keywords and phrases to try and optimize on your web page pages.