Some of these controls are very cool - I've not looked at how "declarative" and easy to use they might be in your own apps but there's a very rich experience going on there.

Intersoft's Silverlight Controls Demo

Take a look at a few of the demos, use the menus as they do a bit more in places than you might think. Performance is pretty good for me as well.

There's also the ones from Telerik that I think I've mentioned before;

Telerik's Silverlight Controls Demo

which also look pretty cool.

Update: Ben mailed me and pointed out the strength of the ComponentOne Silverlight controls so I thought I should link to those as well having played with their control gallery;

ComponentOne's Silverlight Controls

Silverlight's control model is a really strong part of the platform and it's great to see these kinds of controls ( along with others like these from Infragistics ) coming together to really flesh out what you can do without writing a tonne of code yourself.

Kind of makes me want to be a Silverlight developer, throw some of this stuff together and build some apps.