Anything that acts as a memorial to something or someone is commemorative — whether it's an action like a charity walk or an object like a war memorial.
Certain commemorative objects are known simply as commemoratives. Think, for example, of the Elvis Presley commemorative stamps your Dad keeps in his sock drawer (he was young once), or the commemorative coins minted for such events as the inauguration of a new President or the moon landing. See late night TV in particular for "special commemorative offers."
