Almost every application development effort today includes at least one assembly that should be or will be hosted by another application. For example, if you're building a Windows application, you can test it with NUnit, and the nunit-gui.exe is your host. Test a web application, and ACT (Application Center Test) or the ASP.NET worker process (aspnet_wp.exe) is the host. And so on.
I that this article inspired you to explore what COM+ has to offer, to visit nunit.org to find out more about this superlative testing tool, and to have no trepidation about debugging and testing elements of a distributed application and hosted assemblies.