Auto-Tune EFX 2 is the easiest-to-use tool for real-time pitch correction.
Voxengo Marquis does any form of mixing and mastering.
FabFilter Pro-C is a plug-in which can compress computer music files
Blue Cat's StereoScope Pro VST is a real time stereo field analyzer.
8eqf is a semi-parametric 8 band equalizer with selectable bandwidth
