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 ...