I started working on a new Umbraco package a while ago that begain its life as a Class Library. It had been a long day and some time into the project I decided I actually needed my project to be a Web Application Project but there was no obvious way to convert between the two. I suppose I could just create a fresh new project and move my existing files into it but that would be too easy.
I opened the two project files in Notepad++ and after some trial and error discovered that if you copy the line below into your Class Library project file beneath the <ProjectGuid> node and save it then re-open your project in Visual Studio you will magically have a Web Application project that you can add your custom user controls etc to.
[code language="xml"]{349c5851-65df-11da-9384-00065b846f21};{fae04ec0-301f-11d3-bf4b-00c04f79efbc}[/code]