• it’s free
• it works on any browser and any OS
• my talks take up very little space, file-size-wise
• I’m not using someone else’s proprietary software or the same old clip art you’ve seen a million times before
• talks are available in the same format online and off
• easy print option with extra space for hidden notes [new!]

There are a few downsides as well

• it’s not totally standards compliant. If you need total compliance, use S5, it’s great.
• you do need to know a little HTML to make it work for you
• Since you’re not using PowerPoint, you may tend towards wordiness.

People have been asking about it, so I’ve decided to make a blank presentation with all the stylesheets and whatnot, available for downloading. It’s just a basic HTML page, two stylesheets, a sample image or two, and a styleswitcher. Try it out, tweak it, let me know what you think.