There's a waves plugin called PS22. Here's the propaganda:
PS22 StereoMaker is a set of tools for creating convincing stereo results from mono source material as well as a set
of true stereo-in/stereo-out processing tools for synthesizing a richer stereo effect from existing stereo material.
It is far more than merely another “pseudostereo” effect. It is a mixdown and remastering tool capable of producing
subtle or dramatic stereo effects of a kind and quality previously impossible - something that will become an
essential tool in everyday stereo production of all kinds.
The idea of synthesizing a fake stereo effect from mono sources is a very old one, but the results have usually been
disappointing and very often virtually unlistenable, giving unpleasant and unconvincing results. StereoMaker’s
tools are unique, and have been designed after decades of research to avoid the problems of previous processes.