To help determine a site's Yahoo! Web Rank value, the Yahoo! Companion Toolbar collects anonymous URL data about sites visited by Toolbar users who have enabled the Yahoo! Web Rank feature and sends this information back to Yahoo!. The Toolbar does not collect personally identifiable information about you (e.g., your name, phone number, email address, etc.).
Ugh... I'm glad I didn't wake up to this... I've uninstalled it - too busy and like the Alexa toolbar. Does it even have an impact on search?