Entirely free. Requires XCode.
Download and install Soundflower.
Open Audio MIDI Setup.
Create Aggregate Device (Audio -> Open aggregate device editor), constisting of:
- Built-in output (or whatever output you need)
Set default output to Soundflower.
Start audio. At this point you should hear nothing.
Open AU Lab (/Developer/Applications/Audio/AU Lab.app).
Add 1 stereo input track, set Audio Device to "Aggregate device". Click "Done".
Now you should hear sound and see level meters move.
In either Audio 1 or Output 1 strip, click Effects drop-down and select Apple Graphic EQ, or whatever else you want!