OpenStreetMap has been around for awhile and provides free geographic data that can be used by anyone. The data is all community created/contributed and that’s why it’s free to use. Other mapping services such as Bing Maps for Enterprise and Google Maps license their geographic data from some other third party and that’s why they cost thousands of dollars per year to use within commercial applications.
You can find examples of using OpenStreetMap with OpenLayers here: http://wiki.openstreetmap.org/wiki/OpenLayers
Using OpenStreetMap with Silverlight
Unfortunately, there aren’t any open-source Silverlight controls like OpenLayers, but you can easily use OpenStreetMap imagery with the Bing Maps for Enterprise Silverlight Control. Here’s an example of how to do this using the current Bing Maps for Enterprise Silverlight Control Beta release: