Twitter can be a very effective tool for enhancing in-class learning. Take a look at this example from Dr. Monica Ranki's class at the University of Texas at Dallas
In order to use twitter effectively in the classroom, you need to be able to have two things: a hash tag that everyone uses and a way to display the tweets. If you're not familiar with hash tags, take a look at this site for a good explanation. For tools to display the tweets, check out the following web based tools:
Check out the Microblogging for Learning page for more ideas.