To my ears the best mono to stereo effect comes from Waves PS22 X-Split plugin, I love it. What it does is spread the frequencies across the stereo field from low to high in a sine wave (or square) pattern, like this:
 

 
 
There are many parameters to tweak to get it sounding just the way you want it, and it's extremely mono-compatible because the frequencies just collapse back to how they were before when you combine them into a single channel. 
 
One of the best things about it is that you can define the lowest frequency at which the stereo effect kicks in, so if you like you can keep the lower frequencies centered and have them open up into stereo as they get higher. You can also "pan" the stereo image from left to right to position it where you like. Great plugin. The Waves S1 Imager is great for widening tracks which are already stereo. 
 
Another good one, and FREE, is the SHEPPi spacial enhancer which simulates Bob Katz's K-Stereo effect. 
 
Both of the above mono to stereo plugins sound especially good on things like single miked acoustic guitar. Really adds a new dimension to the sound. 

sharke
To my ears the best mono to stereo effect comes from Waves PS22 X-Split plugin, I love it. What it does is spread the frequencies across the stereo field from low to high in a sine wave (or square) pattern, like this:

That picture is a great summary of the things you can do.  Must be an interesting plugin.  I'll be playing around with constructing something similar out of the various more basic parts I have access to.

Paul P

sharke
To my ears the best mono to stereo effect comes from Waves PS22 X-Split plugin, I love it. What it does is spread the frequencies across the stereo field from low to high in a sine wave (or square) pattern, like this:

That picture is a great summary of the things you can do.  Must be an interesting plugin.  I'll be playing around with constructing something similar out of the various more basic parts I have access to.

Yeah it's a fantastic plugin but I'm not sure I would have paid the retail of $150 for it, however you can probably say the same about most of the plugins in the Waves bundles. I paid $300 for the Gold bundle in a sale and given how many plugins came with it I'm guessing I probably paid about $10 per plug, lol.