I've been using Prezi for a while now, using it for classroom presentations and presentations at conferences. 

I found Prezi very easy to use and fun. You can easily create a presentation from scratch, adding text, images, video, links and more. It is very easy to modify them all also with just a click. You can also import slides from a PowerPoint file. The presentations are very dynamic and you can really make them more visually engaging and interesting than PowerPoint or Keynote. It also moves people away from those dreaded bullet points. 

The site also has great help resources, including videos, a help site, and more. There is a community of users on Twitter, Facebook, and the Prezi forum also. 

You can get free accounts for educators or the public. There are limits, but they haven't affected my use at all.

You can create your Prezi's and present them online, or download them for offline use. You can also use Prezi desktop software (with a paid account) to create Prezi's offline.

There is even Prezi Meeting, where you can edit and show Prezi's with up to 9 other users. Work collaboratively on a Prezi or use it as an online meeting platform. 

It's easy to get started and there are a ton of Prezi's that you can look at as inspiration. 

You can make your Prezi's private, public (viewable by everyone) and public with the ability to copy. 

Prezi is fun, easy to use, and create's some great presentations. Try it out. 

Example of one of my early Prezi's (overview shot): (done with minimal experience with Prezi)

Here's the link to the actual Prezi: Google for Educators 

READ MORE - Prezi - the zooming presentation editor

Sliderocket is something I've heard about but never used until today. It is a standalone site but also is a web app in the Google Chrome OS Store, so I started trying it out.

Sliderocket is a free online presentation software (similar to an online version of PowerPoint or Google Docs) that has some really great features. You can create a new presentation or even upload an existing PowerPoint presentation to the site. You can easily add photos from your library or from the internet with Flickr as a choice. Videos are simply installed and embedded. Slide transitions are supported along with allowing viewers to comment on each slide. You can embed Twitter feeds as well as a variety of other features.

Sliderocket has a very good demo/tutorial that walks you through the steps of using it. There is also a good help section and plenty of people using it that post information online.

I found it easy to use and very powerful and will be using the Chrome OS webapp version quite a bit now.

It's very useful for education. Students can interact and collaborate on presentations. Teachers could post presentations and have students comment on the slides. The presentations are also more interactive and have some cool features that PowerPoint doesn't have.

You can even integrate SlideRocket into your Google Apps account.

There is a free version with some limitations and a fee-based version with more features.

READ MORE - Sliderocket - online presentation software