RescueAE is a free VST stereo-imaging plugin that packs a punch. It has a more specific application than most regular plugins and as such is probably best suited to slightly more advanced users.
The plugin works on a stereo channel, but instead of treating the left and right channels independently like you might expect, it encodes them to give a ‘middle’ and a ‘side’ channel. The middle channel is the mono part of the audio, everything that is identical on the left and right channels and sits in the middle of the stereo image. The side channel contains the rest of the audio, everything that is not in the center of the stereo image.
Dividing a stereo channel into ‘middle’ and ‘side’ like this gives you very precise control of the stereo image, which is useful, but RescueAE has a few more tricks up its sleeve to take it from being just useful, to a must-have plugin.
The main two sections of RescueAE are the middle (‘Mid’) and side panels. They both have independent gain controls and can be muted independently, so you can hear clearly the changes made to one section at a time.