For scientific-like presentations (i.e., "clear ans structured" rather than
"fancy"), Beamer LaTeX rules. If you already know LaTeX there is little more
to know. You can see a recent presentation I made and its source to
understand the syntax:
http://dcc.ufmg.br/~lcerf/tutorial.pdf
http://dcc.ufmg.br/~lcerf/tutorial.tar.bz2
If you want, you can use Impressive for transitions and other fancy effects
(to highlight part of the slide) during the actual presentation (Impressive
is a PDF reader): http://impressive.sourceforge.net
For marketing-like presentations (i.e., "fancy" rather than "clear ans
structured"), take a look at Sozi (plugin for Inkscape, the best free
vectorial image editor): http://sozi.baierouge.fr/pages/10-about.html
All those applications are readily available in Trisquel's repositories.
