TweetCloud is a handy application for seeing your top Twitter topics. Here is an example of my last month's tweet topics -- makes sense, given last month was the California School Library Association Conference.
How to use it? One way is to use a TweetCloud to make sure you are staying focused and not tweeting about off-topic subjects. Another way to use it is to capture fun word clouds for a blog, website, or newsletter article -- just tweet about a topic for a day and make a one-day TweetCloud (rather than a month or so).