Powerpoint on the Web without Paying Microsoft?

Yes.  

If you're comfortable hand-coding html or using a tool that can be massaged to take the stylesheet of your choice, there's a completely XHTML/CSS solution.

You make some rudimentary pages.  When they're linked to a document definition, styleshseet and javascript, you get ordered slides--even with click-and-unfold outlines.

The reflexive demonstrations is here:  http://www.w3.org/Talks/Tools/Slidy

A nice primer is here:  http://meyerweb.com/eric/tools/s5/primer.html

--E. R. O'Neill