What can be seized in a seach warrant?

A search warrant must particularly describe the items to be seized. During the course of serving the warrant, while searching for the particular items to be seized, if the officer finds items that he has probable cause to believe are contraband or evidence of a crime, he can seize those items as well.

The scope of the search is a relevant factor. If the search warrant is to locate stolen cars, and the officer finds drugs while looking inside a desk drawer (where you cannot reasonably expect to find a stolen car), that evidence would likely be excluded. However, if while looking for a stolen car, the officer enters a garage (which is covered by the warrant) and inside he finds a pile of stolen license plates on the floor of the garage, those would be seized and would not be excluded.