Backtweets is a very simple interface and goes about this opposite from most of the link services for Twitter. In Backtweets you enter a whole URL string and it returns the users and when they tweeted the link.
It is useful when looking for others that talked about a topic wihtout using the same URL tracking services. Use their own URL as an example. They alo embed the ability to add Backtweets as a search option to your Firefox browser search sites in the upper right.