Very cool. Called Little Red Caps, these have lots of uses out in the shop, sealing open tubes of most anything, sealing the opening of pneumatic tools, covering the ends of gas and hydraulic lines, they have a lot more listed on their site and I bet you can think of about 50 more uses yourself. (Gotta wonder about just when the guy thought this up) They’re available at hardware stores all over, see the long list. Also available online at Lee Valley Tools.
Link via Cool Tools