Matt Cutts: New startup someone should do

You could probably start with a plugin that uses the tweetmeme API - http://help.tweetmeme.com/2009/04/07/api-documentation/ - it'd give you url count, comment count and date created.

-- the other direction you could go in, is start tracking everything a person shares, bookmarks and likes on all their social networks, and aggregate that into a person's public profile. (unfortunately friendfeed doesn't give such info on a url like tweetmeme) From there, use that information for the plugin above, when assessing a url.

You get something similar to this with the bit.ly sidebar bookmarklet. When you shorten a URL, it tells you how many clicks there have been on all bit.ly-shortened links for that URL. I always check out this number, and if a link has over a certain amount of clicks, I usually won't tweet.