There are a lot of private enterprise applications built on it. Its strength was that you could build complex tools relatively quickly. For example, I know of a local company that builds law enforcement software on it. I believe Netflix only uses a small portion of the 'Smooth Streaming' technology in Silverlight.

Also the Feynman lectures on physics. It’s a shame that they went through the effort to put it online only to release it in an inaccessible format. http://research.microsoft.com/apps/tools/tuva/index.HTML

As I recall, recent X Games and Olympics had all their online video content accessible only through Silverlight players also. I remember trying to watch some of these events and being annoyed because the Silverlight player was locking up Firefox.

It ran (past tense) MLB.com's streaming games, but they abandoned it in favor of Flash.

Playboy