A nice visual summary of some locations to get free icons. Of course, always check the licenses of any images you’re using.
(Although the site talks about Adobe Flex, there’s nothing specific about the icons and Flex, they could be used as Silverlight Icons or WPF icons or on a web page).
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Check http://www.ArtistsValley.com for some extensive XAML and Silverlight Icons at a Great price!