I was recently at the MobileX conference in Cincinnati and in preparation for the conference I told the organizers to check out Twitterfall, which they did in a big way, putting it up on a side screen in the auditorium.  It was awesome, and although I just made up the 237% cooler figure, it is truly a powerful way to make your events interactive.  For example, in my keynote I referenced Twitterfall for realtime tweets to be able to work audience questions into my presentation in a seamless fashion.  It's also great for introverts (you know who you are!) who want to ask questions but don't want to disrupt the event by raising their hands.

Here's a video showing how the experience was for attendees: