Umeboshi are Japanese ume — a fruit like an apricot or plum — that are picked while green, then pickled with sea salt for a few months. Brace yourself before popping one in your mouth, though. The gumball-sized hangover cure is exceptionally sour and salty, plus rather bitter. If you can't stomach the taste, try steeping one in green tea before eating [source: Paull].
Umeboshi supposedly work at combating hangovers because their acids can do everything from improve your liver function to assist your digestive system and knock back fatigue. Additionally, they're loaded with electrolytes like sodium and potassium — electrolytes you lose during a drinking binge — and thus help replace them. Finally, their briny, sourpuss flavors make you want to drink a lot of water. Since the main symptom of a hangover is dehydration, drinking a lot of water will definitely help you recover more quickly [source: Paull].
You can find umeboshi for sale all over Japan.