Beamer
Beamer is a LaTeX package designed specifically for creating presentations.
Sample presentations
DanielWatkins gave a tutorial presentation on Beamer; the TeX source is available, as well as the TeX source for the accompanying handout1. For PDF versions of these documents, and some more sample presentations, see DanielWatkins's wiki page.
PresentationTemplate.tex is a template to help with producing presentations for WUGLUG meetings.
Links
1 All of these source files are released under the GNU General Public License.