I use them as fermenters. Generally the easiest way I found was to take off the gas plug and take out the gas dip tube. I then just stretch a piece of 3/8" inner diameter silicone tube over the threads, and then push an airlock into that.
It is a lot easier than it sounds. I thought I had a pic around here somewhere hmmm
"People should not be afraid of their governments. Governments should be afraid of their people." - V
Secondary: Shady Lord RIS, Water to Barleywine, Pumpkin wine, burnt mead
Kegged: Crappy infected mild
Bottles: Apfelwein, 999 Barleywine, Oatmeal Stout, Robust Porter, Robust smoked porter, Simcoe Smash