While at PDC09, I met up with Tor Langlo and Robby Ingebretsen who had just built a really cool closet configuration application using WPF and 3D.
Think about going to a high-end home store, where the consultant sits down with you and helps configure a walk-in or other type of closet for your house. They help you do everything from pick materials to structural layout to deciding where you might want drawers
vs. hanging baskets. Now imagine that they're doing this all in 3d instead of an old pad of white paper with all the options listed and a little spot of graph paper in the corner.
This is not your typical business application, but it still does the things business apps all do including some nice forms over data parts shown near the end of the video. If you're looking for a little inspiration for your own apps, check this out.