Every time I present on Silverlight I make a point to make sure people check out download and install the Silverlight Toolkit. After installing the developer tools, this should be your next step?
Why you ask? Controls! Controls! Controls!
In order to ship more frequent updates of controls and not be as closely tied to the runtime and SDK releases, Microsoft has selected to use CodePlex as the delivery method for all of these addition controls. (Please check out the list of controls below) The site also acts as a feedback mechanism to allow input from you and I on bugs and requested features. The source code is even available!
There are many great controls to use right out of the box, making that much more productive day 1 with Silverlight.
Now go download! http://www.codeplex.com/Silverlight/
[Details from the CodePlex site]