Known users from DebianScience listed in parenthesis.
Conference tools already packaged in Debian
Slides
LaTeX
latex-beamer (upstream) The beamer class is a LaTeX class that allows you to create a beamer presentation. It can also be used to create slides.
prosper Prosper is available for legacy users. New users are recommended to use latex-beamer, which even has a prosper compatibility mode.
You may find the UK TUG FAQ answer on slides helpful.
Posters
This discussion on the mailing list http://lists.debian.org/debian-science/2008/05/msg00158.html contains a number of alternatives (which should ultimately be listed here).
LaTeX
a0poster (part of texlive-latex-extra) Support for designing posters on large paper.
textpos (part of texlive-latex-extra) Place boxes at absolute positions.
ITE (492651)
You may find the UK TUG FAQ answer on posters helpful.
Scribus
Scribus Open Source Desktop Page Layout
Inkscape
Inscape vector-based drawing program. Inkscape loads and saves a subset of the SVG (Scalable Vector Graphics) format, a standard maintained by the WWW consortium.
Impress
Impress OpenOffice.org office suite - presentation
Conference tools that could be considered for inclusion in Debian
Conference tools that cannot go to main for licensing reasons
yorick-mpeg, a Yorick plug-in for writing MPEG1 files, is patent-encumbered. Get it from the unofficial repo.