When you create your own custom control or want to add a control from a DLL that contains one or more controls, to use the control by just dragging and dropping it onto an ASPX page is to add it to your Visual Studio Toolbox. The easiest way to do this is to open your "slide out" toolbox in Visual Studio. It should looks something like this:

When building a custom / web / composite control in Visual Studio 2008 SP1, once you have a custom property that you attempt to set in design mode, you will see an error similar to this in design view: There was an error rendering the control. [value] could not be set on property [property name] The way to duplicate this error is to set a property from the designer on your control, then right-click the control and choose "Refresh" or just close the hosted ASPX page and re-open in design view. Either way, you will see this error.

Just spent the morning getting Service Pack 1 of Visual Studio 2008 installed and configured. These instructions should work for anyone who is installed the Service Pack but there are a few gotchas, especially on Vista that you need to remember when you install.