You can use Google’s Realtime search feature to track trending topics on social media networks like Twitter.  To find it, just do a search as usual, then click on the More icon on the left to open up the Realtime option (or just go to http://google.com/realtime).

The default view will show you the most recent items for the given term(s).  You’ll also see a timeline showing the frequency with which the term(s) have appeared over a given time period.

If you’re hosting an event (like the Stir Trek developer conference in Columbus, Ohio) and have set up a hashtag for the event, this can be very useful to see how things are going on the day of the event:

Now, what I haven’t found is a way to put some kind of scale on the Timeline, or how to do a search query that tells you how many results there were on a given day (please post a comment if you know how to do so).  That would be useful information to plug into your own metrics when assessing your online marketing and social media strategy.  But still, seeing the pretty graph and being able to go back in time and see what people were talking about is pretty darned cool.