Keeping an eye on mentions of your brand / company on Twitter is important, and pretty easy to do, with the range of Twitter search tools available, but one measure of your success on the site is the number of retweets you are getting.
I've just come across a useful tool (though it has been around for a few months) that allows you to see how many retweets you are getting on Twitter, as well as tracking the most popular Twitter users.
Developed by Mike Sheetal of UltraSuperNew, Retweetist tracks and ranks Retweets; I've been trying it out...
The home page of Retweetist tracks the most popular retweets over both the past few hours, and the last 24, in two separate lists. Clicking on a URL will give you more detail about the number of retweets, who retweeted it, and when, which helps understand how it became so popular.
In this example from Retweetist, the popularity of an article on email marketing seems to have been the result of a RT by Mashable, something which this blog has benefitted from in the past, with a post on measuring social success becoming one of our most popular.
It also shows the most popular Twitter users, measured by the number of retweets they receive:
What is useful about the tool is that you can use it to check your own Twitter accounts to see how many RTs you are getting, and who is retweeting your updates. Here's the Econsultancy page, which shows the number of RTs per day for the past 30 days, as well as a list of the most recent:
Have a look at Mashable's Retweetist page to see why a retweet from Pete Cashmore is so valuable; its Twitter account often gets more than 1,000 retweets per day.
Though the ability to dig deeper into your own RTs and track particular URLs would be good additions, Retweetist is very useful for detecting trends, finding popular users, and seeing how your Twitter account is performing in terms of retweets which, as this graph illustrates, is how stories can quickly spread on the site.