The .NET compiler is no big help on this, which is understandable - it can't emit warnings on apparently unused public classes within a class library, as they are most likely part of the library's public API, but happen not to referenced inside the library itself. The same is true for IDE-integrated refactoring tools like Resharper. Resharper points out private/internal methods never called and private/internal types never referenced, but public classes are another story.
So my next bet was on static code analysis tools. They usually let you define the system boundaries, hence it should be possible to identify classes never referenced within those boundaries.
FXCop was one of the most widely used tools in the early days of .NET, but seems a little bit abandoned now, and did not have any matching analysis rule (or at least I didn't find any).
Total .NET Analyzer on the other hand looked very promising and supposedly includes this feature. In contrast to FXCop it parses the sourcecode as well, thus has the means for a more powerful breakdown. Unfortunately it ran out of memory when scanning our Visual Studio solution on my 2GB developer workstation.
Finally I ended up applying NDepend. NDepend has extensive code analysis capabilities, including the highly-anticipated search for unused classes. It also calculates all kinds of other metrics. I have only scratched the surface so far, but what I have seen is very convincing.
