I got a project o mix.  The drum kit is well blanaced onto a single mono track.  What plug in would work best to stereoize the set?

Here's (sort of) what I do:
1) Copy the stereo drum track to another 2-3 channels (stereo tracks). Leave the one with bass/kick/snare/punch in the original track. Leave that stereo track Mono. Roll off some of the high-end (cymbals) if you need).
2) On the first copy, hone in on the hi-hat frequency. Cut out as much of other frequencies as you can reliably. Then, pan it left or right to taste. Do this copy again in other tracks for other cymbals and toms, since not all cymbals and toms are the same exact tone. Keep in mind, the original track will contain much of the left-over frequencies of these panned tracks.
3) On the second copy for general stereo-izing, set one channel slightly out of phase, if you can, by slipping the left or right channel of that track to about 90-degrees out of phase. This is optional.
4) If you have other cymbals or toms you wish to pan, you can make more copies of the stereo track and bell-filter to those frequencies, cutting out all else.
 
Work with this for a bit. In all honesty, the stereo mix of drums is far preferable, but there are some frequency, phase and delay tricks that can stereo-ize a mix. You can also use Convolution reverb to fill out the room a little.
 
Some tools to try as well: Nugen Audio Stereo-ize plug-in and Waves PS22.
 

I'm not an expert, but Craig Anderton wrote an article on the Cakewalk Blog about using TH2 to take a mono recording of piano and converting it to stereo using TH2 that might work for you.

What would be the benefit, since you can't pan the instruments and the balance is already good?
 
EDIT: I read br's extensive reply and can't help but think "phase issues galore". Would this really help any instead of creating more problems? Personally I wouldn't go further than a stereo room plug, probably.

"best" is always a subjective term  You could go through the process brconflict outlined but even his post hinted at the fact that you may run into phase issues.  I would start with some form of widening plugin but if that did not yield results, i would probably recreate the individual drum parts using melodyne and then

Yes, there are phase issues that can be problematic. Some of them are resolved by not going a full 180-degrees out. It's not a total science, but mild use of more than one technique can produce some subtle stereo-izing with good results.
 
This is certainly not a normal occurrence for me. The only reason I ever get a mono drum mix is if it was recorded with only two mics (overhead and kick), or if the original mix-engineer used mono plug-ins on a stereo buss, nullifying the spread.
 
If there's other mics in use, I would certainly try to go back to the source and grab all those tracks for a remix. That's 100 times easier.

Channel Tools might be worth a try since it offers a lot of control. 
 
There's also a simple stereo widener in the Blue Tubes bundle.
 

Maybe MStereoGenerator would help. It is free to try.

You may wish to use this  http://www.nugenaudio.com.roductdetails.php?pid=23

You can also keep it mono, and use an artificial room (short delays) to give a stereo spread. This would leave a lot of room for the other instruments.
 
Kick and snare would be mono anyway, and they're the most important parts. You could use a crossover (e.g., Sonitus multiband set for no compression) to separate out the highs, but I think you're better off building a good stereo ambience if the balance among the drums is good.
 
Stereo wideners (like the Blue Tubes one) will work, but always check for mono compatibility.