KMC 12 Speed Missing Link - Single
Chains can take a real hammering while you ride and sometimes they break. It's worth carrying a spare quick link so you can sort yourself, or a friend, out when this happens. KMC Missing Links compatible with the equivalent SRAM, Shimano and Campy chains.
Here's a single KMC Missing Link for 12 Speed chains. KMC says that this link is not re-usable - and they do not offer a re-usable version yet.
Some riders even swear by carrying two of them :)