I have already covered some of my favourite workflow tips in EQIII and this one is another handy time-saver. By shift-clicking on a control point in the EQ graph in EQIII it is possible to invert the gain of that band.

One of my favourite uses for this is building a false stereo effect on mono sources by creating a suitable "ripple" EQ curve in a multi-mono EQIII plug-in, unlinking the two channels and inverting the bands on the other channel. Instant pseudo-stereo with guaranteed mono compatibility.