A new service, Retweet.it, aims to create an exchange for retweets: for every two tweets you retweet from users of the site, you get one RT in exchange. In other words, it's doing for retweets what TwitterHIT did for Twitter followers. Personally, I'm not a fan.
Retweeting is supposed to be about sharing content because it's interesting and your followers will get value out of it. Rwtweet.it changes that motivation: now you're rewarded for tweeting stuff that might be of no interest to you or you followers: you're just doing it for the reciprocation. Just like with TwitterHIT, I think it's a dangerous precedent: if such services get traction, the term "RT" will go from meaning "here's a valuable tweet" to simply another form of Twitter spam.