Oh, and I guess it's not obvious from title, right now this is Ruby only, but open to ideas and requests to make it cross language, or to a plugin architecture to make that easier
Pretty much SonarQube is the only tool I have found and it's somewhat annoying as a few plugins are commercial and very expensive. As you say, a plugin architecture would be great.
I personally feel that in a few years time Code Analysis/Technical Debt tools will become standard as CI servers have.
And for the doubters, of course no tool is going to tell you very precisely the true Technical Debt of a project but knowing how the code base is changing over time in terms of code count/complexity/todo count, etc I think is something every development team (+ management) should be pay attention to.