Miso soup(味噌汁) is a traditional and most famous in soup for foreigners in Japan. Miso, fermented paste of soy beans, is used for other dishes but definitely mostly used making miso soup.
You can put many varieties of ingredients inside the miso soup. May be most common one is tofu. Then, wakame seaweed is also popular. Also many kinds of vegetables, especially root vegetables such as potatoes, Japanese white radish "Daikon" and onion. Also eggplant can be found inside the soup.
Also seafood such as crab can be the ingredients to make the taste of the soup richer. On the other hand, meat is not a common ingredient for miso soup except pork which is used for TONJIRU, pork soup. Tonjiru also uses miso but it is not considered as miso soup among Japanese.