This should allow someone to do the following:
<a href="http://www.google.com" ping="http://cow.neondragon.net/click.php?google">click me!</a>
When a user clicks on the hyperlink, they will be taken to Google but a request will also be made to http://cow.neondragon.net/click.php?google in the background.
This new attribute can have several good uses. Advertisers can track the popularity of an advert without obscuring the destination URL. You can also track the popularity of outgoing URLs. This can allow you to adjust your webpage, for example making more visited links larger or more prominent.
The Web Applications 1.0 specification says:
When the ping attribute is present, user agents should clearly indicate to the user that following the hyperlink will also cause secondary requests to be sent in the background, possibly including listing the actual target URIs.
Firefox 2.0 plans to do this by displaying URLs which will be pinged in the status bar.