I find it fascinating how the technology has changed over the years. granted, I'm a bit of a maverick - I selected Astound to use for presenting way back in 1998 because it was cross-platform and way ahead of Powerpoint in terms of animation and video playback. I've used one of the tools mentioned below for a couple of years, and am planning on checking out the other soon.
Both use the "freemium" model - accounts are free for basic functionality, and advanced features are available for a subscription.
Slideshare.net allows you to upload presentation files in several formats - Powerpoint, of course, but also Keynote. Others can easily view them and navigate through the presentation in a web browser. The coolest feature, though, is the "Screencast" - where you upload the presention into Slideshare.net, upload an mp3 file to a web-enabled location, and then use an intuitive interface to link the two together.
Here's an example of quickie presentation I threw together for my Mercer class - I was out sick, but needed to go ahead and cover the material.
PREZI.com is something I just found this week. It allows easy creation of ZOOMING presentations, which are visually interesting (as long as they are designed well!).
One of the creators of Prezi has one called Why You Should move Beyond Slides that clearly demonstrated the capabilities and advantages. It is worth checking out.