..but I haven’t seen it yet! How did I get it?
Update
The project is a WCF project. When F5 is pressed in most circumstances, you simply see a webpage for the entire project (showing that it is running in the background). However, per the solution below, if your mouse has a .svc file highlighted before you press F5, it will launch the very slick WCF test client – saving you from having to build your own.

I can’t remember how to get it run when you hit F5, but you can go to:
Microsoft Visual Studio 10.0/Common/IDE/WcfTestClient.exe
Edit:
Aha! I also found how to get it to run when you hit F5. Right click on any .svc (or .xamlx I think) file and click “Set As Start Page” and whenever you run the Host project, the WCF Test Client should also run.