glslDevil is a GLSL debugger http://www.vis.uni-stuttgart.de/glsldevil/

I just gave this a try. Unfortunately when it runs my app the app will at some point crash, and the console will briefly display what went wrong but then the console exits. I can't figure out if the debugger kept a log somewhere.

Anyway, this seems closed-source. Are there any open-source equivalents?

Nevermind, it became open-source at some point: http://glsl-debugger.github.io/

As with so, so many packages, I've never gotten it to run on OS X.