Prezi is a web-based presentation tool using a map layout and zooming to show contextual relationships which addresses some of the shortcomings of Microsoft PowerPoint.

Prezi Public accounts are free for use, and require no annual fee. Each Public account is limited to 100MB of presentation storage space. Public users also have access to the Prezi offline player software, subject to the end user license agreement associated with that software.

EDU Enjoy: Teachers and students receive a special account with 500MB of presentation storage space. Users also have access to the Prezi offline player, subject to the end user license agreement associated with that player. This account is free for anyone with valid education credentials.

Here is a great tutorial by Ryan Cox from St. Cloud, MN.