Prezi is probably one of the most novel presentation tools on the web, and makes quite a splash, if your audience isn't familiar with it. Instead of being based around "slides," a Prezi is laid out on one giant "page," and the window zooms around from view to view:
It takes a bit of getting used to, so be sure to play around with it some before you decide to use it for anything mission-critical.
As with most "freemium" software, some features might be missing unless you pay some money for a Pro account. You'll have to decide what features you can stand to do without, and what's worth slapping down a little cash for.
It can also be a little difficult to find the free stuff. Here's a direct link to create a totally free account. Free accounts are public, so anyone can see and search for your stuff.
If you sign up with a school email address, you can get the "Enjoy" plan for free. This lets you keep your presentations private, for what that's worth. Do you have a school email address? Yes you do, for as long as you're here. Register as firstname.lastname@example.org, and check your email by signing into Gmail with your school account.