I love using Prezi, although it has taken me a while to get used to it, I am starting to use it more and more instead of PPT. What I like about it is the ability to 'zoom' into a presentation. It makes it very clear that we are going into more detail when you zoom further into an area. It can be tricky to get that balance between zoom around to wow your crowd and causing nausea. But Prezi have some good tutorials to help you when you make your first Prezi.
As a teacher I can join Prezi with my school email and I get to have an 'Edu Enjoy' account, which gives you the same perks as an 'Enjoy' account (500MB storage, own logo, private Prezis) but for free. Students can also do the same.
If you haven't come across Prezi before, then here is one of my Prezis (I used this to introduce the project my Yr 9's did for homework). As a testament to my lack of coding knowledge, I think this might be flash based (goodness knows I had fun trying to find out how to embed it). So amusingly it won't work on iPads!
There's also a Prezi app for iPad, it only allows you to view Prezis and do very simple editing. This came in handy when several of my pupils chose to use Prezi to present their projects, I could download their Prezis to my iPad, and then view them away from my computer as I marked them (admittedly in front of the TV).