If you feel that Microsoft PowerPoint or Apple Keynote is not the tool you are most comfortable with, why not use OmniGraffle as a presentation tool?
Creating presentations is as simple as using a canvas for each of the slides you want to have in your presentation.
As most of the computer screens and projectors operate in the landscape mode, you can save yourself a lot of headache by setting up your OmniGraffle document in the landscape mode. This is achieved by using the File | Page Setup... menu command (the corresponding keyboard shortcut is ⇧+⌘+P).
You can start your OmniGraffle presentation by using the View | Start Presentation or by using the ⌥+⌘+P keyboard shortcut command.
OmniGraffle will now go into full screen mode.
You stop the presentation mode by pressing the Esc key.
You can navigate between the canvases by using the arrow keys. Another way of doing the same...