Primary fermentor for tiny batches or for making yeast starters.  Also great as a secondary fermentor for 1 gallon batches.

For a drilled stopper, size 6 or 6½ will work.  Size 6 is a bit on the small side buy inserting an airlock will stretch it out a bit.  For a solid stopper you definitely want a 6½ because there won't be anything stretching it.  For making a yeast starter you should consider a foam stopper. 

Use these for bottling still (non-carbonated) drinks but not for carbonated beverages.  They're not designed to withstand pressure.  Of course you'll read about someone getting away with it; they won't burst every time.  But eventually you'll be cleaning up broken glass and beer.