Wave Arts Panorama, $199
As mentioned above, there are some processors designed to give the illusion of a three-dimensional sonic vista using only stereo channels. Panorama is such a plug-in - using specialised DSP technology, it can give the illusion that a specific sound is placed to the left, right, up, down, front, or back, near or far from the listener. Pretty convincing, and great fun to use.
Read Wave Arts Power Suite 5 review
Flux:: Stereo Tool, Free
It doesn't get much simpler than Stereo Tool, a plug-in that's designed to give you visual feedback of your stereo image. You can control the left and right level individually, invert the phase for each channel, and tweak global stereo pan and width. An essential tool for the mastering engineer, but also quite useful for any stereo tracks at mixdown (think drum overheads or stems).
Download Flux:: Stereo Tool
Brainworx Music bx_solo, Free
Designed as a means by which to accustom yourself to the intricacies of mid-side mic techniques, bx_solo allows you to solo each of the various elements of a mid-side recording. Left and right, mono sum and stereo difference channels are all available, and they're phase-corrected, too. A stepped mid-side stereo width control is included as well.
Download Brainworx Music bx_solo
Cableguys PanCake, Free
As you'd expect from Cableguys, PanCake is a wacky, customisable, over-the-top stereo panner - it's autopanning taken to loony extremes. Instead of the sine or triangle wave LFO seen on traditional autopanners, PanCake lets you create your own waveforms. Up to ten can be stored, and you can sync the lot of them to your DAW's tempo.
Download Cableguys PanCake