There are no slot machines in Las Vegas casinos that can be played without making a wager, but it is technically possible to play a machine for free using free play. Some casinos give a small amount of free play time just for signing up for their rewards program.
Free play is a function of the casino's player's card, which is inserted into machines while gambling to track play for comps. Some casinos give a small amount of free play time when a new player's account is opened, typically amounting to $5. Free play may also be included as a bonus with a hotel stay.