I generally use Keynote for presentations these days, but I have flirted with various LaTeX-based methods of generating presentations in the past. I've just come across a really smooth one called LaTeX beamer. It generates great PDF files with live navigation links, and is very easy to use. Of course, it doesn't generate anything quite as fancy looking as Keynote slides, but some might consider that a benefit.
LaTeX slide packages can be a bit awkward to handle for complex images and so on (and you obviously have to draw figures in another package), but you can't beat them for tiny file sizes, portability1, and the fact that your source files are just plain text so you'll never be unable to open an old presentation again. It also makes it fairly easy to convert your presentation into a suitable format to distribute as printed handouts, or on the web. If you have some experience with LaTeX, and do any public speaking, LaTeX beamer is well worth a look.
1 The PDF files can be displayed by almost any PDF viewer application. I've found that they work just as well on Apple's Preview as on Adobe Acrobat Reader.