As part of my research into Silverlight I looked at what pre-existing controls sets are available for use; to that end I came across and have been experimenting with the Silverlight Toolkit. Much like the Ajax Control Toolkit, the Silverlight Toolkit contains made to measure controls and themes that can instantly be used in your Silverlight applications to improve both the look of the application as well as the user experience. Be careful when downloading this though, it caused me some major headaches getting the March 2009 release to work, I found it better to work with the December 2008 release. Check your Microsoft SDKs folder in Program Files to find your files, this is the default install location for March 2009, for December 2008, you can store them wherever.
The Toolkit features controls such as AutoComplete, TreeView, DatePicker, Calendar, Pie and Bar charts (this is part of a whole data visualization package)and many others are all available. In addition to the controls, the toolkit also contains themes which act much like skins in ASP .NET 2.0/3.5 and allow you to provide a consistent look to all the controls in your XAML file. I enjoyed ShinyBlue and Twilight themes, as well as the Creme theme.