There are a lot of control vendors in the Silverlight Space, with more coming on at an amazing clip. Telerik has been around since 2002 (Ancient History in .NET Terms!) and has been involved with the Silverlight platform since its very inception. I believe the key factors that distinguish Telerik from other vendors are the very high quality of their control suite offerings (I have used both the Silverlight and the ASP.NET AJAX control suites), and secondly, their almost fanatical attention to product support for developers - currently they have a staff of 220 professionals, over 375,000 searchable forum posts, over 80 monthly blog posts, 90+ Telerik Bloggers, and over 312,000 forum members.
In addition, Telerik has Demos,Sample Applications, Video Tutorials, Webinars, Documentation, a Knowledge Base, a Code Library, and a Telerik Trainer. When I had a minor issue related to installation, the email response was fast and complete. To learn more about Telerik and the free license giveaway visit EggHeadCafe.com at
http://www.eggheadcafe.com/
