For the past 8 months or so I've been using the free Orban Loudness Meter to do metering on arbitrary audio signals on my Mac. It has pretty good support for snooping on the real audio interfaces I have connected to my machine. And when its built-in snooping doesn't work, I'll route audio through it using SoundFlower.
The UI isn't pretty but it is by far and away the best loudness meter I've used, including paid options. It's meant for professional metering with an audience of radio and television post-production audio professionals.
It implements some of the new weighted average standards in metering that are designed to get The Loudness Wars under control -- they're weighted such that maximal limiting of the signal ends up showing an overall lower output average power. It encourages mixing for dynamics instead of mixing for loud.
Their Mac 2.0.6 version has been running just fine for me on OS X 10.9.x Mavericks.