Prezi is an online presentation-maker that takes PowerPoint to the next level. Rather than going through a presentation slide by slide, Prezi lets you lay out your presentation on a visual canvas and then move, rotate, scale, and zoom through it.
For some ideas on how you might use Prezi in your classroom, take a look at this Prezi by Paul Hill.
The basic version of Prezi is free to use, but they also offer a Student/Teacher license that gives you access to some of the pro features.
Update: Prezi has added some new features!