Noisebud has released Steijing (Staging), a channel balancing plugin aimed at mastering. You can adjust +-2dB of the left, right, mid and the side channel.
Like many of the Noisebud plugins, Steijing was born out of need. Johannes Ahlberg, mastering engineer and part of the Noisebud development team, needed a quick and easy way to adjust the balance between the left and right channel and couldn't find any product that did it the way he needed it to be done, in small steps with a high-resolution knob that really represents what you're doing with your audio and work great with touch screens.
Available for 32 & 64-bit Windows VST.