Demijohn Bung - Bored w/ Hole for Airlock - Cork
Fits a standard 5 gallon demijohn.
Stick it in the top of your demijohn along with your airlock and protect against infection or oxidation of your brew.
Also fits many fermentors as it's a widely used standard size.
Be aware that you can't effectively sterilise cork, really, so it is advised to replace your cork bungs on a very regular basis.