Seville or sour oranges are perfect for making marmalade; their natural sourness works as a nice foil to the large amount of sugar used when making this jelly. Seville orange juice is an excellent acid for cooking. Cocktails and salad dressings, in particular, can benefit from the greater range of flavor you get from sour oranges as compared to lemons or limes. This fruit tends to be small and has a short window of availability starting in December and ending at the start of February.