With Visual Studio 2008 you can use the IDE as environment for your custom applications!
Basically there are two types of modes to use the VS 2008 Custom Shell:
With integrated mode you merge your application into Visual Studio. You can think of it the way as Team Suite works. There are several Team Editions, like Database Professional, Tester or Architect. Each of these is a seperate module, which runs side-by-side with other modules within the same shell. This way you can integrate your custom tool into Visual Studio to be used parallel with other tools and the VS standard toolset.
If you want your application to run seperately from Visual Studio, isolated mode is the best choice for you! Just use the common shell and custom brand your application. This way you can use the infrastructure of the devenv.exe shell (like toolbars, windows,..) but use your custom display logic and even rebrand the entire application.
Well - this really seams to be an interesting new possibility to me! Extend Visual Studio or just reuse the shell (and ship it royalty free within your app). No need to rebuild such a client with third-party controls any longer!
Actually this would have been very helpful to me at least once - where I built a management client for a business workflow..
I'm really curious about Visual Studio 2008 Beta 2 (which ScottG says will ship this week) - because it will include the first public version of Visual Studio Shell.
I think the usefullness depends on the size and customizability of the shell.