Twitter know lets you see what the trending topics are on your Twitter home page. However, many of us do not have a clue what these topics actually are.
What the Trend? to the rescue! This application (by Matt Mayer) explains what the topics are, when they started and the last time they were tweeted about. If you click on a trend, you can see the tweets, any news or photos (from Flickr) about the subject as well.
You can also follow @whatthetrend on Twitter.