StereoFX is a universal binary AU plugin and one of the best stereo widener plugs out there. It is a stereo expander so center images remain stable, and it widens far-field information like reverb better than anything. Now with mid-squash. Mono-friendly. StereoFX is $59.99.
Be aware, though, that it's not designed to change the stereo image radically- this is a mastering-style widener at its best when adjusting stereo subtly. If you push it very hard, stereo content will develop a distorted quality, so don't try to use it in place of mixing adjustments. StereoFX is for widening (or bass mono-ing) meant not to be obvious.
This is StereoFX on a full mix- Dry | StereoFX
