Here's a direct link: https://code.google.com/p/leak-finder-for-javascript/

Would it be possible to run this against an ASP .Net app with lots of JS? It refers to Python in the documentation, so I'm not sure.

Only if you wrote your JavaScript in a very particular way while using Google's libraries.

I'm curious, has anyone actually used Closure as a javascript project for any major site outside of Google (not hobby project)? I've heard people talk about using the compiler, but never the full package.

I don't know of any, to be honest. However, Closure Library is really just part of the equation. It becomes much more powerful when used with all of the Closure Tools.

I have tried the closure tools for my own internal uses and while I don't particularly expect myself using it for all my projects, it definitely shines on bigger projects. It is a pretty good set of tools.

We at http://www.universalsubtitles.org/en/ have. The code is open source https://github.com/pculture/unisubs/tree/dev/media/js/widget . I'm writing a blog post about that should be up still today on our development blog. TL;DR It's full of good code, some interesting idea. But, tries to make Javascript feel like Java (annotated types, very verbose etc). Also makes it difficult to use other libs (specially with the compiler in advanced mode). I don't think it's a good option for most teams.