How can I quantify the amount of technical debt that exists in a project?

Question

Does anyone know if there is some kind of tool to put a number on technical debt of a code base, as a kind of code metric? If not, is anyone aware of an algorithm or set of heuristics for it?

If neither of those things exists so far, I'd be interested in ideas for how to get started with such a thing. That is, how can I quantify the technical debt incurred by a method, a class, a namespace, an assembly, etc.

I'm most interested in analyzing and assessing a C# code base, but please feel free to chime in for other languages as well, particularly if the concepts are language transcendent.