This is like asking "do drugs work". It's too vague to answer. There are a whole bunch of drugs people refer to as supplements and they all have different structures and different effects.
St. Johns Wort can be an effective antidepressant just by itself, for instance. Piracetam has rather mild nootropic effects that vary from person to person and with dose, etc.
Here's one of our nootropics threads, that should get you started. Search for "nootropics" or the name of the specific supplements on Google too.