Boz Digital Labs have recently released a free plugin: Width Knob.
Control Stereo Width with Width Knob
With the plugin you can quickly and easily adjust the stereo width of an aux bus or track.
The plugin does so by panning alone.
Note that it doesn't enhance the stereo image or anything like that, that's something you may achieve through M/S Processing.
Dan Cooper at Pro Tool Expert took it through its paces:
You can download Width Knob here.
Just add it to your shopping cart and checkout. You'll need an account at Boz.