Backtweets is awesome

Tweetmeme has been doing that for eons. You can even include it as a chicklet on your article, to encourage more retweets.<p>See, for example, <a href="http://tweetmeme.com/story/767267088/danieltennercom-how-to-nap" rel="nofollow">http://tweetmeme.com/story/767267088/danieltennercom-how-to-...</a>

I like their API - been using it in <a href="http://tweetbe.at/" rel="nofollow">http://tweetbe.at/</a> to bulk follow or list everyone who references a url

Ubervu has this feature too.