Well after a bit of research I found out what designers probably already know - it turns out they're called ferrules. These examples are from Paxton Hardware if you happen to be in search of some, although I'm sure other companies carry them. Paxton has an informative guide for measuring the size you need here.
I think ferrules give a nice finishing touch to certain types of furniture. Love them on this Sasha side table by Alexa Hampton for Hickory Chair.
On another note, don't forget to plan on going to the DC Design House which is taking place April 10 - May 9th. I've heard from a reliable source that it's going to be terrific!