Often when working with WPF and Silverlight, it is difficult to create line of business applications using only the main SDK controls and toolkit controls. There may be occasions when you want a powerful control that is neither available in the SDK nor in the Toolkit. In such cases, if you can afford it, you should go for third party tools. There are several companies today providing state of the art, rich, and robust controls for all such needs. Here's a list of websites of each company that provides an amazing set of controls. While a few controls may be free, the good ones are usually accessible upon paying.
Check out their websites to know the latest offerings and prices:
http://telerik.com
Telerik
http://componentone.com
ComponentOne
http://www.infragistics.com
Infragistics
http://www.DevExpress.com
DevExpress Controls
http://cellbi.com
Cellbi
http://ComponentArt.com
ComponentArt
http://ComponentOne.com
ComponentOne
http://FarPoint.com
FarPoint
http://Xceed.com
Xceed
http://Vectorlight.com
Vectorlight
http://Visifire.com
Visifire