tweetmeme is my new toy for watching what’s going on in the Twitosphere. http://tweetmeme.com/. It allows you to watch for any terms and get stats for the last day, week and month. I particularly like the categories, where you can see which Twitter topics are recently popular.
If you search for something like #RPattz — a recent trending topic as today is the boy’s 23rd birthday — you can find out which of the millions of inane tweets about Robert Pattinson were the most popular. This is the most useful feature, I think, of the site.