Soy sauce contributes both saltiness and a kind of meaty, or umami flavor. The substitute you choose will depend inpart on why you want to replace it. To reduce sodium, try using light, or low sodium soy sauce. But if you are allergic to soy try this:
Soak dried mushrooms in just enough warm water or beef broth to cover; allow to soak overnight. Add a little Worcestershire to the soaking liquid after removing the mushrooms.