Keynote was the first iWork application. It was first used by Steve Jobs in his presentations to developers and the public at MacWorld conferences. Not only was it designed as a powerful communication tool, but it also showcased Apple's technologies and commitment to sophisticated design.
Today, Keynote is better than ever. If I had to pick one feature that I like most, it would be the presenter's display. Most of us who speak in front of groups plug a projector into our laptop so that people can see the slides on a screen (or a white board in some cases). Uses the Mac's built-in management of multiple screens so that you can easily configure the output to the projector to show the slides and the laptop's screen to show the customizable presenter's display. I generally configure the presenter's display to show the current slide and the next slide along with the time. The laptop is in front of me, and the screen is behind me, so that I can talk to the audience and see what they're seeing without turning around. And that clock is a gift. There's nothing worse than attending a presentation where all you see is the speaker's back -- except when he decides to check his watch. People come up to me sometimes after a presentation and ask how I do it.
But that's just my personal favorite. There are so many more features in Keynote that make presentations easier to prepare and easier for people to watch and participate in. iWork '09 for Dummies covers them all.