Prezi is an online presentation tool that dispenses with the idea of individual slides. You get one huge canvas on which you can add text, images, and shapes. You scatter these elements anywhere you want on the page, and then build a path that you can use to "fly" through the presentation. There's never a page change or a "next slide, please" moment -- it's all one big seamless zoom around an infinite virtual backdrop. I don't say this sort of thing often, but the effect is simply stunning.
Most of the common stuff you'd expect to find in a presentation package is here. You can add hyperlinks, images, video, and sound. You can size and rotate text, add bulleted lists, and more.
Prezi does its magic using Flash in a browser window, and you can use it for free, or pay for a premium account ($59/year or $159/year, depending upon how much storage space you need) that removes the Prezi watermark, adds privacy settings, and even includes an offline editor.
But don't take my word for it. The easiest way to understand Prezi is to check out a presentation, like this one: