what are some alternatives to LibreOffice Impress?

There's Calligra Stage. I've never used it, but it's available in the repo (just search "Calligra").

> Have you used one of the alternatives? How did it compare to Impress?

You can use LaTeX with Beamer too. When document is exported as PDF,
you shouldn't have any surprise (in the layout) during the
presentation. But it is less user-friendly, of course (especially when
you never uses LaTeX).

HTML + javascript presentation tools:

https://trisquel.info/es/forum/free-software-getting-betterand-faster-it-seems#comment-46781

For scientific-like presentations (i.e., "clear and 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 and 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.

Thanks a lot guys, these all look great. I'll give them a try, and add a few to the documentation.