Queen Excluder - Metal Bound
Not every hive needs a queen excluder but it's a handy tool to have in the shed. If you're going to get one, get a good one.
If your queen starts laying in your honey supers, put this on long enough for the bees to emerge and the cells to be filled with nectar - but be careful to locate the queen first and make sure she's below the excluder.
Note for beginners: when your hive is drawing wax on new foundation in a honey super don't add a queen excluder right away; bees don't like to build wax above it.