Beamer
Beamer is a LaTeX class that allows you to create a presentation like PowerPoint. However, unlike Powerpoint, the slides are fixed pdfs. Among other benefits, this ensures that your mathematical symbols won't randomly turn into wingdings in the middle of your talk, making you look like a complete tool.
It's also much snazier than Prosper, and a bit more complicated (but not by much).
I like Beamer a lot. I've created an unofficial CSIRO style file, which can be downloaded from the links to the right.
Here's a screenshot.
We should all use Beamer.