Exposé and dashboard during a Keynote presentation!
There is a great feature that you can enable in Keynote that will enable Exposé and Dashboard while a presentation is running. It is in the Keynote preferences, under "Slideshow". You can now switch to another application during a presentation and when you switch back to Keynote, it will resume exactly where you left off. It is also possible to use this to refer to a Wikipedia article in dashboard without interrupting anything.