Twitter always had a functionality to mark a tweet as “favorite”. However, I personally never even bothered to use it that often because it just didn’t have any good use. I only used it more like a “tweet bookmark” tool to save certain tweets which I wanted to come back to later.
But it looks like that Favstar.fm is going to change the way Tweet Favorite is being used. Favstar is looking at twitter favorites more like a social vote. So basically their main home page currently shows tweets that has been voted (a.k.a favorite’ed) a certain number of times. If you sign in via your Twittter login information, favstar also gives you a whole range of functionality, including the ability to follow people from directly from Favstar.fm and see what tweets your friends are favoriting. Also, by signing in it is very easy to see who is favoriting your tweets which in my opinion is an excellent insight for any twitter user.
Apparently there is another service called Favard that does something similar.