I bought a fifth of vodka to bring to a party tonight. (Why, you ask? They said to bring something. And there often comes a moment towards the end of the evening where I'm thinking "damn, these people should have bought more vodka".) A fifth used to be a fifth of a gallon; now it's 750 mL. A fifth of a gallon is 756 mL, so the name makes sense. I'm kind of curious as to why this is a traditional unit, though. How Many?: A Dictionary of Units of Measurement (which is tremendously interesting, especially when you have other things you should be doing) says that it's an American version of a traditional British unit called the "bottle", which was one-sixth of an Imperial gallon, or 758 milliliters. The fact that an Imperial gallon is very nearly six-fifths of a U.S. gallon is apparently a coincidence.
While I was at the liquor store I noticed that they also sold vodka in 375 mL bottles. To my surprise, this is not called a "tenth". There are 28,800 google hits for "fifth of vodka" and 2 google hits for "tenth of vodka". The aforementioned unit dictionary mentions that a "half bottle of champagne" (375 mL) is called a "fillette".
Some people believe that a fifth and a liter are the same thing.
Another common size is the 1.75-liter bottle, often called a handle, and that size makes no sense at all; it's not two fifths, it's not a half-gallon, it's not all that round of a number in the metric system. I would have expected the "larger" size to be 1.5 liters (two fifths) or 2 liters. I thought it might be something to do with European law, but European law says that spirits must be sold in bottles of 0.02, 0.03, 0.04, 0.05, 0.10, 0.20, 0.5, 1, 1.5, 2, 2.5, or 3 liters. Notice that neither .75 nor 1.75 appears here. From what I can gather, the reason for these laws is so that a company can't decrease the size of their bottle slightly and sell it for the same price, so price hikes are actually visible to the consumer. 0.75 liters is on the list of allowed bottle sizes for wine, though.