Ethyl acetoacetate, also known as ethyl 3-oxobutanoate (FEMA# 2415, CAS# 141-97-9), is only very sparingly found in nature. It has been reported in coffee, passion fruit and strawberries, but I suspect it is only an occasional, extremely minor, partner of its much more abundant relative ethyl 3-hydroxybutanoate.
The most obvious uses, by far, are apple and strawberry flavors, but this useful ingredient is capable of adding the illusion of authenticity and realism to a very wide range of different flavors.
One small word of caution needs to be added regarding the practical use of ethyl acetoacetate. The common practice of adding a little water to propylene glycol solvent should be avoided if ethyl acetoacetate is used. Even relatively slow, small-scale reactions between ethyl acetoacetate and water can generate significant amounts of carbon dioxide, possibly enough to break a glass bottle.