Post Made Community Wiki by Adam Shiemke
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Umami comes from a very specific source: Glutamates and glutamic acid. MSG is a glutamate, so it is one of the best ways to add a bit of umami.

Meat, Kombu seaweed (used to make Dashi, and hence Dashi), mushrooms, onions, cheeses, soy and other beans, most high-protein foods.

Here is a quick list. Most are prepared foods, but it should give you an idea.