Many other sites have sprung up to track what's hot on Twitter: we covered many of these, including the excellent Tweetmeme, in our recent article HOW TO: Retweet on Twitter (worth a look if you're getting up to speed on what retweeting is).
Today we were contacted about another new service in this space: DailyRT seeks to rank tweets by the number of retweets they've received: it's basically Digg for retweets. It supports Sign In with Twitter, so you can retweet right there on the site, and also tracks retweets of Twitter trends. The key feature, however, is one many trend sites lack: the ability to search and find top retweeted topics from the past. When you log in, you can also save these searches.
If you're looking to see what's hot on Twitter, these new sites are a huge help: DailyRT is a very competent effort and definitely one to watch.