Via these posts:
Free Silverlight Controls And Tools For Brighter Websites
Silverlight 2 UI Templates
Following are useful links to visit before writing Silverlight applications.
1) XAML Power Toys
XAML Power Toys is a Visual Studio 2008 SP1 Multi-AppDomain Add-In that empowers WPF & Silverlight developers while working in the XAML editor. Its Line of Business form generation tools, Grid tools, DataGrid and ListView generation really shorten the XAML page layout time.
2) Silverlight Community Gallery
2.a) Silverlight 2 Community Samples
3) XAMLTemplates.net
Control templates available for purchase
4) Blacklight
Blacklight is a UX focused code sharing project. Microsoft has released a bunch of technologies that allow designers and developers to work closely together to make beautiful software. This project is a collection of controls, samples, visual assets and ideas that has been put together by User Experience designers and developers to both show you what the technology is capable of (from a UX point of view), and give you code and samples that you can use in your own projects, completely free of charge (see the License tab above). Check out the showcase to see what is currently available...
5) Dave's Glossy Controls
This is a set of easily customisable control templates for the following controls
6) Free Silverlight Controls And Tools For Brighter Websites
A must see, includes much of what is listed here.