Wow!  My boss found this yesturday - I think it's a new feature in VS05 because I've never seen it before.  I'll be really annoyed if it's been in VS03 and I just never found it.

When you create a winform user control you normally need some container to test it in.  In the past I've always just done this as part of the larger application.  Now, when you have a control library project in VS, you can hit F5 and it will launch the UserControl Test Container:

Even better, you can load froms from any dll or exe:

Very cool!  This is a lot like the old ActiveX test container - but way nicer to use.