Simple but quality frequency shifter delay by ValhallaDSP: http://www.valhalladsp.com/valhallafreqecho
Mac/PC VST/AU/RTAS. Think it's 32/64 bit I've been using in 32-bit.
Can't stretch the frequency too far without it sounding unnatural but don't think natural sounding stuff is what it's intended for.
Btw I also agree who whoever mentioned in this thread that single/a few recommendations with descriptions are better than linking to whole lists/compilations, even if they are all good. Hopefully this will take a little bit of the legwork out of things. On that note, can anyone recommend a decent VST phaser for Mac?
Cheers