This is a list of all the 3rd Party controls that have demos or samples running on the latest version of Silverlight.

ComponentOne	A full suite of components including custom layout panels, menus, treeview, textboxes, charts and more.
DevExpress	A free datagrid with grouping, sorting, resizing, template support, multi-row selection and more.
Infragistics	A large collection of charts and gauges for data visualization.
Intersoft	Two alternative controls to the standard menu, FishEye and CoverFlow.
NETiKA Tech	An implementation of the standard Windows Forms control library, developed as Windows Forms Applications and run as Silverlight applications.
Telerik	A full suite of components including menus, tabcontrols, treeviews, mediaplayer, progressbar and more.
Visifire	A large collection of open source data visualization components.
Xceed	An asynchronous, multiple file upload control with data compression.

I'll keep this page up to date as Silverlight evolves and control offerings grow.

Silverlight 2 has just hit Beta 2 and they are already some really nice offerings available.  Take a minute and check out some of the cool demo applications on each site that show off the controls.