NuGen Audio have released a couple of free VST (Windows) plug-ins available for download via their site. The pair of plugi-ins, MonFilter and Stereoizer LT and offer control of mono and stereo sources.
Monfilter LT
Monofilter provides a simple interface designed to convert a stereo signal below a specified frequency into a mono signal. The balance knob allows adjustment of the remaining stereo signal above the cutoff frequency. Useful for ensuring that sounds are suitable for mastering to vinyl and keeping a check on LF signals in general, especially where stereo effects processing has been applied.
Stereoizer LT
Stereoizer adds 'stereo spread' to a mono signal by splitting the frequencies into bands and then shifting them differently in the R and L channels. Great for adding depth to percussion loops and livening up synth pads etc
To get yours, shimmy over to: