Google is a relative newcomer to the web-based presentation scene, but Google Presentations is now an integral part of the broader Google Docs suite, which also includes word processing and spreadsheet tools.
The conversion from .ppt into Google is very pretty—allowing for the customary flattening of animations, media, etc. Editing the resulting presentation works more smoothly than at Zoho or ThinkFree. As at these latter sites, you can also create a Google presentation de novo. The interface is intuitive and responsive, and the predesigned themes are passable.
Perhaps the strongest suit of the google product is its collaborative functions. All three of the major web-based presentation tools offer co-authoring, but Google lets you edit simultaneously.
When it's time to publish your show, you can make it visible to the world using a distinctive url—here is mine—but you cannot easily embed it, as with Zoho or Slideshare. If the viewer is logged in to a Google account, he or she is supposed to be able to chat with other viewers or participate in a live presenation, but I have not been able to make this work. The collaboration tools will be great for working with peers, but they are less useful for sharing your presentations with your classes. You can give students a link to your show, but you cannot easily put it right on your own page.
In the end, Google is also bound to benefit from the fact that Google accounts are becoming de rigeur in some student circles, especially given initiatives such as Google Apps for Education, which is, incidentally, also available here at Wesleyan.