While this may not be approved of by the production company -- or be downright illegal -- props may get taken home by the cast or crew after the filming is wrapped. Typically these are smaller items, such as hats, memorabilia or knickknacks. Some may end up on online auction websites to create a small amount of side income for the seller. However, some extremely valuable props, such as a missing Batman movie costume, have been found in employee's homes.
Some props are sold off at auctions afterward. These are more valuable items, like cars or jewelry, and have a collector appeal to them. This helps the studio regain a small amount of cost for collecting or building the prop pieces. Additionally, local restaurants/specialty businesses may buy at these auctions to display the item at their business.
Some of the rarer props may be put in boxes and stored in the large prop warehouse that the studio runs. The props that are selected for storage are entirely up to the discretion of the prop master/executives of the film. Once in storage, a prop may eventually be re-used again, or it may not. If a prop was rented from an independent prop house, it is sent back.
Finally, props may end up in the garbage. This includes small items like cups, shoes and keys, but occasionally may include larger items. A studio will throw props away if it feels it cannot use them again, if it feels the item has no resale value, or if it simply doesn't have enough time to set up an auction. That doesn't mean that the prop thrown away isn't interesting; many collectors feel the trashed props still have resale value to the right customer.