Can law enforcement seize items that are not listed on their search warrant?

The best answer is "it depends."  Normally, when the police enter a house to search for items, they are limited to parameters of the warrant.  Keep in mind, however, that many warrants are written very broadly with extra language to the effect that they may search for "x" items and "any other evidence of the commission" of the crime.  This broad language gives the investigator very wide latitude to seize anything closely related to the offense.  This is how officers are often able to scoop-up extra items.

If the items is not evidence of the offense identified in the warrant, then the officer may still seize an item if it is in plain view and immediately recognizable as contraband.  For example, if an officer is running a search warrant to look for stolen property and sees a bag of marijuana on the coffee table as they enter the residence to serve the warrant, they may seize the marijuana as contraband seen in "plain view."  Plain view is when an officer sees something in his sight in an area that he has the right to be.  Even if the officer is in the closet looking for stolen credit cards, but finds marijuana in view of where he was looking, he could still seize the contraband because he had the authority to be in the closet looking for the property.