The Moth

I've seen introductory sessions where people jump straight into Visual Studio and/or Blend and start demonstrating how to build a Silverlight application. IMO, Silverlight is a relatively new technology and there are potentially people that have not seen examples of applications built with it. As such it is important to demonstrate its capabilities first by running some applications and clicking around. There are many samples to choose from the Showcase and from the Gallery, so take your pick.

In my sessions I show my favourites (i.e. the ones that appeal to me personally, for one reason or another).

1. Hard Rock Memorabilia (shows the DeepZoom feature in a public released site)
2. Silverlight Airlines (gets you thinking of great user experience for booking a flight)
3. Cameras (another example of great user experience for shopping online scenario)
4. Image Snipper (shows graphics and interaction you would only expect from a desktop app)
5. Video Puzzle (combination of a game with video elements)

...then before jumping into code, it is worth showing some of the controls that are available and also how they can be skinned (templated):
6. Control Demo (not all of them, but gives you an idea)
7. Corina's control skins Flat, Bubbly, Red, Rough.

...if you are looking for a sample that combines skinning, the OpenFileDialog, IsolatedStorage and also throws in a bit of extensibility, do check this out:
8. Calculator (get its story from the author's blog post)

Finally,
9. a great business application that is almost indistinguishable from a local client and has only recently been made available is the Healthcare Demo (built in the UK).

What publically available Silverlight application(s) float your boat?