Tweetmeme has been doing that for eons. You can even include it as a chicklet on your article, to encourage more retweets.

See, for example, http://tweetmeme.com/story/767267088/danieltennercom-how-to-...

BackTweets has actually been around for a while too -- and was always a part of BackType Connect (which shows you other reactions including hacker news, digg, reddit, friendfeed). We even personalize results to show a viewer which of their friends shared a link: http://www.backtype.com/connect/danieltenner.com%252fposts%2...

We have a similar chicklet to encourage retweets; it differs in that the colors are completely customizable and it integrates automatically with google analytics. Another one of our widgets shows only the interesting tweets: http://widgets.backtype.com/backtweets?url=http://danieltenn...

I like their API - been using it in http://tweetbe.at/ to bulk follow or list everyone who references a url

Checking out your app. very cool idea.

There's a semantic problem though. I DO NOT like how you treat the people I'm following as a list that I need to add people to. It's really confusing. If you're going for the gmail comparison there should be a way to check off people and then click a button that's separate from the list drop down that says "Follow them." As is, I found it really counterintuitive.

Yep good points - in the code I abstracted vanilla followers to just be another list, but I think almost everyone considers their followers to be something really different than a list.

Also, please don't hide the logout button under the @username dropdown. I had to try about 3 guesses before I found it.

Ubervu has this feature too.