I've started loking for good SilverLight / Windows Presentation Foundation examples.
Here are some I've found so far:
DENOUNCE
Podcast Listener and Blog Reader
NOSTALGIA 


Yahoo! Flickr Browser and Photo Manager
DA.CODE 

Interactive Trivia Game
CINE.VIEW 


Netflix for Your Desktop
Harmony: visualize your music library 

One impressive feature of Windows Vista is its support for 3D. This proof-of-concept is a knock-off of Coverflow (recently integrated into iTunes) with a few extra "twists".
de.collage: 13 painters with a view 
In French, "décollage", roughly translates to "take off" or "to become unstuck". This application queries the internet to bring famous paintings to your desktop.
Module: the right hand for your desktop 

Can't wait for Microsoft Vista's Sidebar? Why not try ours? Simply launch the application, type in your zip code, and away you go.
Echobox: adding voice to design
Design is hard to manage by email. This proof-of-concept facilitates basic design collaboration via FTP.
fp.hue: color conversion for Sparkle 

Sometimes you have to make your own tools. This widget converts HEX color values into Sparkle's floating point RGB values.