On March 18, 2004, Yahoo! released what they called Yahoo! Web Rank. Yahoo! explained it as follows:
Yahoo! Web Rank is the name that Yahoo! has given to a technical measurement of a particular URL's popularity. If you choose to enable the Yahoo! Web Rank feature on the Yahoo! Companion Toolbar, a toolbar icon will display the Yahoo! Web Rank value of each URL that you visit. The Yahoo! Web Rank values range from 1 to 10. This feature is currently in Beta release.
That page that described it used to be at http://help.yahoo.com/help/companion/webrank/ but now that page redirects.
You can take a look at the Wayback Machines copy of it from April 1, 2004. Lots of good old details on it.
Even Danny Sullivan wrote a huge write up on it at Search Engine Watch back on April 2, 2004.
Where did it go? Why did Yahoo! stop using it?
Forum discussion at DigitalPoint Forums.