John McCain's most powerful ad -- an open letter to Barack Obama in which an Iraq War veteran criticizes his stand on the war -- didn't cost the Republican candidate a dime. In "Dear Mr. Obama," army veteran Joe Cook stands in front of the camera and scolds the Democratic presidential candidate for calling the Iraq war a mistake. The two-minute video, which was posted on YouTube, has gotten more than 11 million hits and is the most popular election video on the site.