Silverlight 3 runtime and SDK introduce many new controls. Some of these controls were previously released as part of the Silverlight Toolkit and some are completely new. Regardless of their origins, since they are now part of the Silverlight 3 product, these controls have full reference documentation that includes art, code examples and links to live samples. These controls enable scenarios such as auto-completion for a textbox, the display of hierarchical data, navigation within pages of a Silverlight application and much more. Here's a list of new controls and dialogs:

AutoCompleteBox (my personal favorite)

ChildWindow

DataPager

DescriptionViewer

Frame

HeaderedItemsControl

Label

Page

SaveFileDialog

TreeView

ValidationSummary

VirtualizingStackPanel

In addition to the new controls, the ListBox control now enables multiple selections using the SelectionMode property.

For a listing of all the Silverlight controls in alphabetical order, see Controls and Dialogs. For a listing of the controls by function, see Controls by Function.