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A GPL would suffice if we had one. However, we do not have an open-source version of our AJAX controls. In order to develop an application you will need to purchase a license or the company you work for to purchase one for you.
A purchased license of our .NET controls includes a run time royalty-free license to redistribute the controls in your own solutions. Your end users/clients will not need to purchase licenses in order to use the applications, you developed using our products, as soon as you comply with the redistribution guidelines in our end-user License Agreement. When you deploy your applications, you have to make sure that the DLL files of the Telerik controls are protected and locked, so that your clients don't have access to them. If your clients are going to update your project via using our controls in design time, this is considered as violation of our License Agreement and they will have to purchase the appropriate number of licenses for our controls.
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed