Angostura Bitters is one of those things that you never buy, but if you like your booze, you almost always need.  Not a lot, but a few drops here and a few drops there, can make the difference between a cocktail that tastes like the base liquor, and something that tastes good enough to be thrown around in a cocktail shaker by Tom Cruise himself.   It never goes bad – because it’s 44.7% alcohol, and preserves itself.