The Silverlight Toolkit is a project built by the Silverlight product team, which contains a lot of "out-of-release cycle" controls.
These are grouped into three parts:
Controls include various nice controls (lots of them already part of WPF):
These controls are especially useful if you plan to write business applications. Also they have similar controls in WPF, so if you plan to port your apps from Silverlight to WPF they come in handy as well.
The second part contains a number of various chart controls.
As third part, the toolkit contains a couple of themes, which can be applied to your controls.
Download the Silverlight Toolkit from http://www.codeplex.com/Silverlight/.