I can contribute describing the differences between Keynote, Impress and LaTeX Beamer. Previous answers only seem to have second hand knowledge of what Keynote can. What makes Apple Keynote "so cool" are the smart guides which let you align stuff with great efficiency and speed. It also handles all kind of media files and is still extremely fast at it. In OpenOffice/LibreOffice you need much more clicks and sub-menus to align graphics or text. Once you have added images which have a slightly higher resolution it becomes unusable and slow to the point that you just hate to work with it because you are spending time on waiting for it to save the document. Trying to embed video files is even worse. The LaTeX Beamer class templates look terrible scientific and are not usable for presentations outside academia and research just because of their graphic design (notable exeption: hsrmbeamertheme has a refreshing look). Sure you could make your own styles, but typically presentations are done with very little time, and you just want to throw some things in, not modify styles just to see how it looks when you have three pictures on one slide instead of one. This is where Keynote shines. Efficiency, ease of use and speed. SCALA was praised in one of the answers here, but is a digital signage solution, I'm not sure if it even compares to presentation software at all.
You can use the Free software Scribus to create your presentations as PDF documents (Scribus nightly now even has smart guides). For the presentation itself I employ the pdf-presenter-console package (unmaintained, but in the repositories) or open-pdf-presenter which even allows for an simple XML file containing the presenter notes. You might also want to check out Impressive for on the fly highlighting and transition effects (but no presenter screen support). PDF presentations generally don't allow videos though, which can be a huge drawback compared to Keynote.
If you are working with video or audio files, then there is a good free alternative: Inkscape with the extensions JessyInk (which comes pre-installed with Inkscape) or Sozi if are into Prezi-style presentations (zooming and panning on one big canvas). Both are solid, you can embed media (audio, video, links) as well as mouse roll-overs and so forth. The files are saved as SVG. Presentations happen in a browser (full screen mode). The types of media files and codecs you can play back depend on the browser you use.