Linux.com's Dmitri Popov presents KeyJnote, "a program that displays presentation slides." Simply put, it is a souped up slideshow app instead of a presentation tool since it lacks the "authoring" part, i.e., you need to prepare your slides, in PDF or other popular graphics format, and get it to process it into a cool presentation. KeyJnote is written on Python making it available on all major platforms. To run the KeyJnote presentation, you need the application installed along with the Python interpreter.
Similarly, you may also want to take a look at Wink, dubbed as "a Tutorial and Presentation creation software, primarily aimed at creating tutorials on how to use software." Compared to KeyJnote, Wink gives you both the authoring and presentation tools in one. However, to take advantage of Wink's power, you need to have Player installed.
Another tool that is worth a look at is S5: A Simple Standards-Based Slide Show System. In contrast to the first two mentioned above, S5 only requires a web-browser for your presentation. Creating your slides, however, requires you to download the template and edit it, either using your favorite text editor or your HTML tool. No other special software needed.
All of these are handy but most of us prefer an all-in-one suite like OpenOffice.org Impress. :) Personally, I prefer doing it in plain vanilla PDF, without all the transitions, etc. How about you?