Favstar.fm allows you to see the most favorited tweets across Twitter at any given time. It also allows you to track your own tweets to see who is favoriting them, and allows you to follow what your friends are favoriting.
I concur with the other reviewers that Favstar is an excellent and very useful app. I use Favstar on a daily basis, checking favorites as well as RTs to get a sense as to which of my tweets are getting the most response/reaction. I should mention that Favstar recently added RTs along with favorites - providing a more holistic view of interest in a tweet.
But what's really amazing about Favstar are it's two other dimensions:
1) Favstar is pretty much the keeper of funny on Twitter - their leaderboard and the @favstar100 and @favstar50 accounts keep track of some of the funniest...