One of the sites I go to frequently is www.silverlight.net/showcase. Theres always new stuff showing up here and it gives me tons of ideas.

The business section is also awesome because it has tons of samples of great applications that should really jog your brain for ideas.

One of the great things about SL and WPF is how we can break the mold of application design and come up with truly great new applications for our   users. That’s one are where the showcase can help to get new ideas on things you can do.