For a full featured solution, including plugins, read on.
[2020-03-14 update]: I have not used this approach for a long time, so I don't know if it's still free and valid. I recommend the BlackHole approach.
Install Sound Siphon. We will use the demo version, which is free and does what we need.
Install AU Labs from Apple. You may need a free Apple Developer Account. To tell you the truth, I can't figure out how to get this.
Launch Sound Siphon
We mute Sound Siphon so the sound doesn't play twice; once through AU Labs and once through Sound Siphon.
Launch AU Lab
Add the equalizer
Save the AU Labs document
Back up the document as AU Labs corrupts it often!