So here at work, we are trying to get some web controls to work with pop-up windows. Here is a nice easy way to set up your test envirnoment.
Create a new website for testing. Eg. WebControlTester.
within the Solution Explorer place your web control into a new folder App_Code
then add a web.config file to the Solution Explorer.
Within the web.config file add:
<add namespace="Your.Namespace.In.WebControl" tagprefix="SomeTagPrefix">
Now you can edit your web control, build it and then switch to the web site and run it.