Gallon glass jugs have a million uses! Many home brewers and vintners use them for fermentation vessels when making small batches of beer or wine. One gallon can make five 750mL bottles of wine or ten 12 oz bottles of beer. When used as fermenters, a #6 drilled stopper is recommended to accommodate an airlock (a #6.5 stopper will work as well).
These jugs can also be used as growlers, for sharing a gallon of your delicious brew with friends. They can also be used as blowoff reservoirs for blowoff assemblies on your bigger beer brewing projects, or for yeast propagation. It's never a bad idea to keep a few of these on hand!